Semiconductor device and method for manufacturing the same

ABSTRACT

An object is to provide a high reliability thin film transistor using an oxide semiconductor layer which has stable electric characteristics. In the thin film transistor in which an oxide semiconductor layer is used, the amount of change in threshold voltage of the thin film transistor before and after a BT test is made to be 2 V or less, preferably 1.5 V or less, more preferably 1 V or less, whereby the semiconductor device which has high reliability and stable electric characteristics can be manufactured. In particular, in a display device which is one embodiment of the semiconductor device, a malfunction such as display unevenness due to change in threshold voltage can be reduced.

TECHNICAL FIELD

The present invention relates to a semiconductor device including anoxide semiconductor and a method for manufacturing the semiconductordevice.

Note that in this specification, a semiconductor device refers to alldevices that can function by utilizing semiconductor properties, andelectro-optical devices, semiconductor circuits, and electronic devicesare all semiconductor devices.

BACKGROUND ART

In recent years, a technique by which a thin film transistor (TFT) ismanufactured using a semiconductor thin layer (with a thickness of aboutseveral nanometers to several hundreds of nanometers) formed over asubstrate having an insulating surface has attracted attention. Thinfilm transistors are applied to a wide range of electronic devices suchas ICs or electro-optical devices, and prompt development of thin filmtransistors to be used especially as switching elements in image displaydevices is being pushed. Various metal oxides are used for a variety ofapplications. Indium oxide is a well-known material and is used as atransparent electrode material which is needed for liquid crystaldisplays and the like.

Some metal oxides have semiconductor properties. The examples of such ametal oxide having semiconductor properties are tungsten oxide, tinoxide, indium oxide, zinc oxide, and the like. A thin film transistor inwhich a channel formation region is formed using such a metal oxidehaving semiconductor properties is already known (Patent Documents 1 and2).

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-96055

DISCLOSURE OF INVENTION

The electron field effect mobility of a thin film transistor in which achannel formation region is provided in an oxide semiconductor is higherthan that of a thin film transistor in which amorphous silicon is used.

Such an oxide semiconductor is expected to be used for manufacturingthin film transistors on a glass substrate, a plastic substrate, or thelike, and to be applied to display devices such as a liquid crystaldisplay, an electroluminescent display device, and electronic paper.

In an active-matrix display device, electric characteristics of thinfilm transistors included in a circuit are important and the performanceof the display device depends on the electric characteristics of thethin film transistors. Among electric characteristics of thin filmtransistors, threshold voltage (V_(th)) is particularly important. Whenthe threshold voltage is high or negative even when the field effectmobility is high, it is difficult to control the circuit. In the casewhere a thin film transistor has high threshold voltage and a highabsolute value of the threshold voltage, the thin film transistor cannotperform a switching function and might be a load when it is driven atlow voltage. Further, in the case where the threshold voltage isnegative, current tends to flow between a source electrode and a drainelectrode even if the gate voltage is 0 V, that is, the thin filmtransistor tends to be in a so-called normally-on state.

In the case of an n-channel thin film transistor, it is preferable thata channel be formed and drain current flow only after positive voltageis applied as gate voltage. A transistor in which a channel is notformed unless the driving voltage is increased and a transistor in whicha channel is formed and drain current flows even in the case of anegative voltage state are unsuitable for a thin film transistor used ina circuit. In the case where variation (the amount of change) in thecharacteristics of transistors included in a circuit in a semiconductordevice is large, a malfunction due to variation in threshold voltagemight be caused.

In particular, in a liquid crystal display device, in the case wherethreshold voltage greatly varies between elements, a malfunction such asdisplay unevenness due to the variation in threshold voltage might becaused.

Further, also in a display device including a light-emitting element, inthe case where on current (I_(on)) of TFTs (TFTs in a driver circuit orTFTs for supplying current to light-emitting elements arranged inpixels) which are arranged so as to make constant current flow to apixel electrode greatly varies, a malfunction such as variation inluminance on a display screen might be caused.

It is an object of one embodiment of the present invention to provide athin film transistor which operates stably for a long time and asemiconductor device in which the thin film transistor is used.

One embodiment of the present invention disclosed in this specificationis a semiconductor device. The semiconductor device includes a gateelectrode layer over a substrate having an insulating surface; a gateinsulating layer over the gate electrode layer; an oxide semiconductorlayer over the gate insulating layer; a source and drain electrodelayers over the oxide semiconductor layer; and an insulating layer incontact with part of the oxide semiconductor layer, over the gateinsulating layer, the oxide semiconductor layer, and the source anddrain electrode layers.

One embodiment of the present invention disclosed in this specificationis a method for manufacturing a semiconductor device. The methodincludes the steps of forming a gate electrode layer over a substratehaving an insulating surface; forming a gate insulating layer over thegate electrode layer; forming an oxide semiconductor layer over the gateinsulating layer; performing first heat treatment after the formation ofthe oxide semiconductor layer; forming a source and drain electrodelayers over the oxide semiconductor layer; forming an insulating layerin contact with part of the oxide semiconductor layer, over the gateinsulating layer, the oxide semiconductor layer, and the source anddrain electrode layers; and performing second heat treatment after theformation of the insulating layer.

Note that the first heat treatment is preferably performed in a nitrogenatmosphere or a rare gas atmosphere. In addition, the first heattreatment is preferably performed under a condition that the highesttemperature in the treatment is higher than or equal to 350° C. andlower than or equal to 750° C. Note that the temperature of a so-calledovershoot portion which is generated at the time of temperature controlis not included as the highest temperature in this specification.

The second heat treatment is preferably performed in an air atmosphere,an oxygen atmosphere, a nitrogen atmosphere, or a rare gas atmosphere.In addition, the second heat treatment is preferably performed at atemperature higher than or equal to 100° C. and lower than or equal tothe maximum temperature of the first heat treatment.

With the above structure, at least one of the objects is achieved.

An oxide semiconductor used in this specification is formed into a thinfilm represented by InMO₃(ZnO)_(m) (m>0), and a thin film transistor ismanufactured using this thin film as an oxide semiconductor layer. Notethat m is not always an integer. Note that M denotes one metal elementor a plurality of metal elements selected from Ga, Fe, Ni, Mn, and Co.As an example, M may be Ga or may include the above metal element inaddition to Ga, for example, M may be Ga and Ni, or Ga and Fe. Moreover,in the above oxide semiconductor, in some cases, a transition metalelement such as Fe or Ni or an oxide of the transition metal iscontained as an impurity element in addition to a metal elementcontained as M. In this specification, among the oxide semiconductorlayers whose composition formulae are represented by InMO₃(ZnO)_(m)(m>0), an oxide semiconductor whose composition formula includes Ga as Mis referred to as an In—Ga—Zn—O-based oxide semiconductor, and a thinfilm of the In—Ga—Zn—O-based oxide semiconductor is referred to as anIn—Ga—Zn—O-based non-single-crystal layer.

Besides the above, the following oxide semiconductors can be used forthe oxide semiconductor layer: an In—Sn—Zn—O-based oxide semiconductor;an In—Al—Zn—O-based oxide semiconductor; a Sn—Ga—Zn—O-based oxidesemiconductor; an Al—Ga—Zn—O-based oxide semiconductor; aSn—Al—Zn—O-based oxide semiconductor; an In—Zn—O-based oxidesemiconductor; a Sn—Zn—O-based oxide semiconductor; an Al—Zn—O-basedoxide semiconductor; an In—Ga—O-based oxide semiconductor; an In—O-basedoxide semiconductor; a Sn—O-based oxide semiconductor; and a Zn—O-basedoxide semiconductor. Silicon oxide may be contained in the oxidesemiconductor layer. Addition of silicon oxide (SiO_(x) (x>0)) whichhinders crystallization into the oxide semiconductor layer can suppresscrystallization of the oxide semiconductor layer at the time when heattreatment is performed after the formation of the oxide semiconductorlayer in the manufacturing process. Note that the oxide semiconductorlayer is preferably amorphous but may be partly crystallized.

The oxide semiconductor preferably contains In, and further preferablycontains In and Ga. In order to obtain an I-type (intrinsic) oxidesemiconductor layer, dehydration or dehydrogenation is effective.

Depending on the conditions of the heat treatment or the material of theoxide semiconductor, the state of the oxide semiconductor layer ischanged from an amorphous state to a microcrystalline state or apolycrystalline state. Even when the state of the oxide semiconductorlayer is changed to a microcrystalline state or a polycrystalline state,switching characteristics as a TFT can be obtained.

A thin film transistor having small variation in threshold voltage andelectric characteristics which are stable for a long time can beprovided. Thus, a semiconductor device which includes highly reliablethin film transistors having favorable electric characteristics can beprovided.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1D illustrate manufacturing steps of a semiconductor device;

FIG. 2 illustrates a semiconductor device;

FIGS. 3A to 3C illustrate semiconductor devices;

FIGS. 4A and 4B illustrate the concentration of hydrogen in oxidesemiconductor layers;

FIGS. 5A1 and 5A2 show HO ionic strength of oxide semiconductor layers,and FIGS. 5B1 and 5B2 show H₂O ionic strength of the oxide semiconductorlayers;

FIGS. 6A to 6F show analysis results of a mechanism in which a watermolecule is eliminated from an oxide semiconductor;

FIGS. 7A and 7B are block diagrams of a semiconductor device;

FIGS. 8A and 8B are a circuit diagram of a signal line driver circuitand a timing chart, respectively;

FIGS. 9A to 9D are circuit diagrams of a structure of a shift register;

FIGS. 10A and 10B are a circuit diagram of a shift register and a timingchart, respectively;

FIGS. 11A1, 11A2, and 11B illustrate a semiconductor device;

FIG. 12 illustrates a semiconductor device;

FIG. 13 illustrates a semiconductor device;

FIG. 14 illustrates a pixel equivalent circuit;

FIGS. 15A to 15C each illustrate a semiconductor device;

FIGS. 16A and 16B illustrate a semiconductor device;

FIG. 17 illustrates a semiconductor device;

FIG. 18 illustrates a semiconductor device;

FIG. 19 illustrates a semiconductor device;

FIG. 20 is a circuit diagram illustrating a structure of a semiconductordevice;

FIG. 21 illustrates a semiconductor device;

FIG. 22 illustrates a semiconductor device;

FIG. 23 illustrates a semiconductor device;

FIG. 24 is a circuit diagram illustrating a structure of a semiconductordevice;

FIG. 25 illustrates an example of an e-book reader;

FIGS. 26A and 26B illustrate examples of a television set and a digitalphoto frame, respectively;

FIGS. 27A and 27B illustrate examples of game machines;

FIGS. 28A and 28B illustrate examples of a portable computer and aportable phone, respectively;

FIG. 29 is a graph for defining V_(h);

FIGS. 30A and 30B show results of a BT test performed on a thin filmtransistor in Example 1; and

FIGS. 31A and 31B show results of a BT test performed on a conventionalthin film transistor.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, Embodiments and Example of the present invention will bedescribed in detail with reference to the accompanying drawings.However, the present invention is not limited to the description below,and it is easily understood by those skilled in the art that modes anddetails disclosed herein can be modified in various ways withoutdeparting from the spirit and scope of the present invention. Therefore,the present invention is not construed as being limited to thedescription of Embodiments and Example.

Embodiment 1

In this embodiment, one embodiment of a method for manufacturing a thinfilm transistor 150 illustrated in FIG. 1D will be described withreference to FIGS. 1A to 1D that are cross-sectional views illustratingmanufacturing steps of the thin film transistor. The thin filmtransistor 150 has a bottom-gate structure called a channel-etchedstructure.

First, over a substrate 100 having an insulating surface, a gateelectrode layer 101 is provided through a photolithography process withthe use of a photomask. Note that a resist mask may be formed by aninkjet method. Formation of the resist mask by an inkjet method needs nophotomask, which results in a reduction in manufacturing costs.

It is preferable that a glass substrate be used as the substrate 100. Inthe case where the temperature of heat treatment performed later ishigh, a glass substrate having a strain point of 730° C. or higher ispreferably used as the substrate 100. Further, as a material of thesubstrate 100, for example, a glass material such as aluminosilicateglass, aluminoborosilicate glass, or barium borosilicate glass is used.Note that when the glass substrate contains more barium oxide (BaO) thanboron oxide (B₂O₃), more practical heat-resistant glass can be obtained.Thus, a glass substrate containing BaO and B₂O₃ in which the amount ofBaO is larger than that of B₂O₃ is preferably used.

Note that a substrate formed of an insulator, such as a ceramicsubstrate, a quartz glass substrate, a quartz substrate, or a sapphiresubstrate, may be used instead of the substrate 100. Besides,crystallized glass or the like may be used.

An insulating layer serving as a base layer may be provided between thesubstrate 100 and the gate electrode layer 101. The base layer has afunction of preventing diffusion of an impurity element from thesubstrate 100, and can be formed to have a single-layer structure or astacked-layer structure including one or more of a silicon nitridelayer, a silicon oxide layer, a silicon nitride oxide layer, and asilicon oxynitride layer.

When a halogen element such as chlorine or fluorine is contained in thebase layer, a function of preventing diffusion of an impurity elementfrom the substrate 100 can be further improved. The peak of theconcentration of a halogen element to be contained in the base layer ismeasured by secondary ion mass spectrometry (SIMS) and is preferably inthe range of 1×10¹⁵ cm⁻³ to 1×10²⁰ cm⁻³.

A metal conductive layer can be used as the gate electrode layer 101. Asa material of the metal conductive layer, the following is preferablyused: an element selected from aluminum (Al), chromium (Cr), copper(Cu), tantalum (Ta), titanium (Ti), molybdenum (Mo), and tungsten (W),an alloy containing the above element, an alloy containing theseelements in combination, or the like. For example, a three-layerstructure in which an aluminum layer is stacked over a titanium layerand a titanium layer is stacked over the aluminum layer, or athree-layer structure in which an aluminum layer is stacked over amolybdenum layer and a molybdenum layer is stacked over the aluminumlayer is preferable. Needless to say, the metal conductive layer mayhave a single-layer structure, a two-layer structure, or a structure inwhich four or more layers are stacked.

Then, a gate insulating layer 102 is formed over the gate electrodelayer 101.

The gate insulating layer 102 can be formed to have a single layer of asilicon oxide layer, a silicon nitride layer, a silicon oxynitridelayer, or a silicon nitride oxide layer or a stacked layer thereof by aplasma CVD method, a sputtering method, or the like. For example, asilicon oxynitride layer may be formed by a plasma CVD method usingSiH₄, oxygen, and nitrogen as deposition gases. The thickness of thegate insulating layer 102 is set to 100 nm to 500 nm inclusive. In thecase where the gate insulating layer 102 has a stacked-layer structure,for example, the stacked-layer structure includes a first gateinsulating layer with a thickness of 50 nm to 200 nm inclusive and asecond gate insulating layer with a thickness of 5 nm to 300 nminclusive over the first gate insulating layer.

Before the formation of the oxide semiconductor layer, heat treatment(at higher than or equal to 400° C. and lower than the strain point ofthe substrate) may be performed in an inert gas atmosphere (e.g.,nitrogen, helium, neon, or argon) so that impurities such as hydrogenand water contained in the gate insulating layer 102 are removed.

Then, over the gate insulating layer 102, an oxide semiconductor layeris formed to a thickness of 5 nm to 200 nm inclusive, preferably 10 nmto 50 nm inclusive. In order to be amorphous even after heat treatmentfor dehydration or dehydrogenation which follows the formation of theoxide semiconductor layer, the oxide semiconductor layer preferably hasa small thickness of less than or equal to 50 nm. The small thickness ofthe oxide semiconductor layer makes it possible to prevent the oxidesemiconductor film layer being crystallized when heat treatment isperformed after the formation of the oxide semiconductor layer.

The oxide semiconductor layer is formed using an In—Ga—Zn—O-basednon-single-crystal layer, an In—Sn—Zn—O-based oxide semiconductor layer,an In—Al—Zn—O-based oxide semiconductor layer, a Sn—Ga—Zn—O-based oxidesemiconductor layer, an Al—Ga—Zn—O-based oxide semiconductor layer, aSn—Al—Zn—O-based oxide semiconductor layer, an In—Zn—O-based oxidesemiconductor layer, a Sn—Zn—O-based oxide semiconductor layer, anAl—Zn—O-based oxide semiconductor layer, an In—Ga—O-based oxidesemiconductor layer, an In—O-based oxide semiconductor layer, aSn—O-based oxide semiconductor layer, or a Zn—O-based oxidesemiconductor layer. For example, in this embodiment, the oxidesemiconductor layer is formed using an In—Ga—Zn—O based oxidesemiconductor target by a sputtering method. Alternatively, the oxidesemiconductor layer 130 can be formed by a sputtering method in a raregas (typically argon) atmosphere, an oxygen atmosphere, or an atmosphereincluding a rare gas (typically argon) and oxygen. In the case where asputtering method is used, it is preferable that film formation beperformed with a target containing SiO₂ at 2 wt % to 10 wt % inclusiveso that SiO_(x) (x>0) which inhibits crystallization is contained in theoxide semiconductor layer to prevent the oxide semiconductor layer frombeing crystallized in heat treatment performed later for dehydration ordehydrogenation. Note that a pulsed direct-current (DC) power source ispreferably used, in which case dust can be reduced and the thicknessdistribution can be uniform.

Further, the relative density of the oxide semiconductor in the oxidesemiconductor target is preferably greater than or equal to 80%, inwhich case the concentration of impurities in the formed oxidesemiconductor layer can be reduced, which leads to excellent electriccharacteristics and high reliability of a thin film transistor.

Examples of a sputtering method include an RF sputtering method in whicha high-frequency power source is used as a sputtering power source, a DCsputtering method, and a pulsed DC sputtering method in which a bias isapplied in a pulsed manner. The RF sputtering method is mainly used forforming an insulating layer, and the DC sputtering method is mainly usedfor forming a metal layer.

In addition, there is also a multi-source sputtering apparatus in whicha plurality of targets of different materials can be set. With themulti-source sputtering apparatus, different materials can be depositedto be stacked in one chamber, or plural kinds of materials can bedeposited by electric discharge at the same time in one chamber.

In addition, there are a sputtering apparatus provided with a magnetsystem inside the chamber and used for a magnetron sputtering method,and a sputtering apparatus used for an ECR sputtering method in whichplasma generated using microwaves is used without using glow discharge.

Furthermore, as a deposition method using a sputtering method, there arealso a reactive sputtering method in which a target substance and asputtering gas component are chemically reacted with each other duringdeposition to form a thin compound film thereof, and a bias sputteringmethod in which voltage is applied also to a substrate duringdeposition.

Preheat treatment is preferably performed so as to remove moisture orhydrogen remaining on an inner wall of the sputtering apparatus, on asurface of the target, or in a target material, before the oxidesemiconductor film is formed. As the preheat treatment, a method inwhich the inside of the deposition chamber is heated to a temperature of200° C. to 600° C. inclusive under reduced pressure, a method in whichintroduction and exhaust of nitrogen or an inert gas are repeated whilethe inside of the deposition chamber is heated, and the like can begiven. In this case, not water but oil or the like is preferably used asa coolant for the target. Although a certain level of effect can beobtained when introduction and exhaust of nitrogen are repeated withoutheating the deposition chamber, it is more preferable to perform thetreatment with the inside of the deposition chamber heated. After thepreheat treatment, the substrate or the sputtering apparatus is cooled,and then the oxide semiconductor film is formed.

The substrate may be heated to a temperature of higher than or equal to400° C. and lower than or equal to 700° C. during the formation of theoxide semiconductor film by a sputtering method.

It is preferable to remove moisture or the like remaining in thesputtering apparatus with the use of a cryopump before, during, or afterthe formation of the oxide semiconductor film.

The gate insulating layer 102 and the oxide semiconductor film may beformed successively without exposure to air. Successive film formationwithout exposure to air makes it possible to obtain each interfacebetween stacked layers, which is not contaminated by atmosphericcomponents or impurity elements floating in air, such as water,hydrocarbon, or the like. Thus, variation in characteristics of thinfilm transistors can be reduced.

Then, the oxide semiconductor layer is processed into an island-shapedoxide semiconductor layer 103 through a photolithography process (seeFIG. 1A).

Alternatively, a resist mask for forming the island-shaped oxidesemiconductor layer 103 may be formed by an inkjet method. Formation ofthe resist mask by an inkjet method needs no photomask, which results ina reduction in manufacturing costs.

Then, first heat treatment is performed to dehydrate or dehydrogenatethe oxide semiconductor layer 103. The temperature of the first heattreatment for dehydration or dehydrogenation is set to 350° C. to 750°C. inclusive, preferably 425° C. or higher. Note that in the case wherethe temperature is 425° C. or higher, the heat treatment time may be onehour or shorter, whereas in the case where the temperature is lower than425° C., the heat treatment time is set to longer than one hour. Forexample, the substrate is put in an electric furnace that is a kind ofheat treatment apparatus, the heat treatment is performed on the oxidesemiconductor layer in a nitrogen atmosphere, and then water or hydrogenis prevented from being mixed into the oxide semiconductor layer withthe oxide semiconductor layer not exposed to air; thus, the oxidesemiconductor layer 103 can be obtained. In this embodiment, slowcooling is performed in one furnace in a nitrogen atmosphere from aheating temperature T at which dehydration or dehydrogenation isperformed on the oxide semiconductor layer 103 to a temperature lowenough to prevent entry of water; specifically, the slow cooling isperformed until the temperature drops by 100° C. or more from theheating temperature T. The atmosphere is not limited to a nitrogenatmosphere, and the dehydration or dehydrogenation may be performed in arare gas atmosphere (e.g., helium, neon, or argon).

Through the first heat treatment, rearrangement at the atomic leveloccurs in the oxide semiconductor included in the oxide semiconductorlayer 103. The first heat treatment is important because the first heattemperature can reduce distortion which hinders transfer of carriers inthe oxide semiconductor layer 103.

In the first heat treatment, it is preferable that water, hydrogen, andthe like be not contained in nitrogen or a rare gas such as helium,neon, or argon. It is preferable that the purity of nitrogen or a raregas such as helium, neon, or argon which is introduced into a heattreatment apparatus be set to be 6N (99.9999%) or higher, preferably 7N(99.99999%) or higher (that is, the impurity concentration is 1 ppm orlower, preferably 0.1 ppm or lower).

The first heat treatment can be performed by a heating method using anelectric furnace. Note that in the first heat treatment, the heattreatment apparatus is not limited to an electric furnace and may beprovided with a device that heats an object to be processed by thermalconduction or thermal radiation from a heater such as a resistanceheater. For example, a rapid thermal anneal (RTA) apparatus such as agas rapid thermal anneal (GRTA) apparatus or a lamp rapid thermalannealing (LRTA) apparatus can be used. An LRTA apparatus is anapparatus for heating an object to be processed by radiation of light(an electromagnetic wave) emitted from a lamp such as a halogen lamp, ametal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressuresodium lamp, or a high pressure mercury lamp. A GRTA apparatus is anapparatus for performing heat treatment using a high-temperature gas. Asthe gas, an inert gas which does not react with an object to beprocessed by heat treatment, like nitrogen or a rare gas such as argon,is used.

In some cases, the oxide semiconductor layers might be crystallized tobe a microcrystalline layer or a polycrystalline layer depending on theconditions of the first heat treatment or the material of the oxidesemiconductor layer. For example, the oxide semiconductor layer might becrystallized to be a microcrystalline semiconductor having a degree ofcrystallization of 80% or more, or 90% or more. Depending on thematerial of the oxide semiconductor layer, the oxide semiconductor layermight be an oxide semiconductor layer containing no crystal.

The first heat treatment can be performed before processing the oxidesemiconductor film into the island-shaped oxide semiconductor layer 103.In that case, the substrate is taken out of the heating apparatus afterthe first heat treatment, and then a photolithography process isperformed.

Here, analysis of the concentration of hydrogen in an oxidesemiconductor layer which was dehydrogenated and an oxide semiconductorlayer which was not dehydrogenated are described. FIG. 4A is across-sectional schematic view of a sample used in this analysis. Theprepared sample was formed as follows: an oxynitride insulating layer401 was formed over a glass substrate 400 by a plasma CVD method, and anIn—Ga—Zn—O-based oxide semiconductor layer 402 was formed to a thicknessof about 40 nm over the oxynitride insulating layer 401. The preparedsample was divided into two pieces: one of the two was notdehydrogenated and the other one was dehydrogenated at 650° C. for 6minutes in a nitrogen atmosphere by a GRTA method. The concentration ofhydrogen in the oxide semiconductor layer of each sample was measured sothat the effect of dehydrogenation by heat treatment was examined.

The concentration of hydrogen in each oxide semiconductor layer wasmeasured by secondary ion mass spectrometry (SIMS). FIG. 4B shows theresults of analysis by SIMS of the concentration distribution ofhydrogen in the oxide semiconductor layers in the thickness direction.The horizontal axis represents depth from surfaces of the samples, andthe point at the left end where the depth is 0 nm corresponds to theoutermost surfaces of the samples (the outermost surfaces of the oxidesemiconductor layers). An analysis direction 403 shown in FIG. 4Arepresents the analysis direction of the SIMS analysis. The analysis wasperformed in a direction from the outermost surface of each oxidesemiconductor layer to the glass substrate 400, that is, in a directionfrom the left end to the right end of the horizontal axis in FIG. 4B.The vertical axes of FIG. 4B are logarithmic axes representing hydrogenconcentration and ionic strength of oxygen at a certain depth of eachsample.

In FIG. 4B, a hydrogen concentration profile 412 is a profile of theconcentration of hydrogen in the oxide semiconductor layer which was notdehydrogenated, and a hydrogen concentration profile 413 is a profile ofthe concentration of hydrogen in the oxide semiconductor layer which wasdehydrogenated by heat treatment. An oxygen ionic strength profile 411shows oxygen ionic strength which was obtained in the measurement of thehydrogen concentration profile 412. The oxygen ionic strength profile411 was not drastically changed and substantially constant, which meansthat the SIMS analysis was performed with precision. Although not shown,oxygen ionic strength was measured also in the measurement of thehydrogen concentration profile 413 in a manner similar to that of theoxygen ionic strength profile 411 and the oxygen ionic strength was alsosubstantially constant. The hydrogen concentration profile 412 and thehydrogen concentration profile 413 were quantified with the use of areference sample formed using an In—Ga—Zn—O-based oxide semiconductorlayer similarly to the samples.

Note that it is known that it is difficult, in principle, to obtaincorrect data in the proximity of a surface of a sample or in theproximity of an interface between stacked layers formed using differentmaterials by the SIMS analysis. It is thought that correct data couldnot be obtained from the outermost surface of the samples to a depth ofabout 15 nm in this analysis; thus, the profiles at a depth of 15 nm ormore were evaluated.

The hydrogen concentration profile 412 shows that hydrogen was containedin the oxide semiconductor layer which was not dehydrogenated atapproximately 3×10²⁰ atoms/cm³ to approximately 5×10²⁰ atoms/cm³ and atan average hydrogen concentration of approximately 4×10²⁰ atoms/cm³. Inaddition, hydrogen concentration profile 413 shows that the averagehydrogen concentration in the oxide semiconductor layer could be reducedto approximately 2×10¹⁹ atoms/cm³ by dehydrogenation.

The analysis confirmed that dehydrogenation by heat treatment allows theconcentration of hydrogen in the oxide semiconductor layer to bereduced. The analysis also confirmed that dehydrogenation at 650° C. for6 minutes in a nitrogen atmosphere by a GRTA method allows theconcentration of hydrogen in the oxide semiconductor layer to be reducedone tenth or less.

FIGS. 5A1 and 5A2 each show H+O ionic strength measured at the same timeas the SIMS analysis shown in FIG. 4B, and FIGS. 5B1 and 5B2 each showH₂+O ionic strength measured at the same time as the SIMS analysis shownin FIG. 4B. FIG. 5A1 shows the H+O ionic strength of the oxidesemiconductor layer which was not dehydrogenated, and FIG. 5A2 shows theH+O ionic strength of the oxide semiconductor layer which wasdehydrogenated. FIG. 5B1 shows the H₂+O ionic strength of the oxidesemiconductor layer which was not dehydrogenated, and FIG. 5B2 shows theH₂+O ionic strength of the oxide semiconductor layer which wasdehydrogenated. It is found that the samples which were dehydrogenatedhad lower H+O ionic strength and H₂+O ionic strength than the sampleswhich were not dehydrogenated and that elimination of moisture or OH wasefficiently performed by the heat treatment at 650° C. for 6 minutes bya GRTA method.

Next, FIGS. 6A to 6F show results of computational chemistry analysis ofa mechanism in which a water molecule is eliminated from theIn—Ga—Zn—O-based oxide semiconductor. A quantum chemistry calculationprogram Gaussian 03 was used for the analysis. In the oxidesemiconductor, as well as a water molecule, OH and H might be eliminatedas a water molecule with OH and H combined with each other; therefore,the elimination mechanism of an OH group existing in the oxidesemiconductor was analyzed.

FIG. 6A shows an initial state of a most stable structure in the oxidesemiconductor including OH groups, and FIG. 6D shows a final state wherethe OH groups become a water molecule (an H₂O molecule) to be eliminatedto infinity. FIGS. 6B and 6C show a transition state and an intermediatestate before reaching the state shown in FIG. 6D from the state shown inFIG. 6A. Symbols M₁, M₂, and M′ denote metal atoms corresponding to In,Ga, and Zn. In other words, there are six combinations of M₁ and M₂: Inand In, Ga and Ga, Zn and Zn, In and Ga, In and Zn, and Ga and Zn. Notethat the calculation was performed with M′ replaced by a hydrogen atomon the smallest molecule structure basis. The step-by-step descriptionon the elimination mechanism of the OH group is given below.

First, in the initial state, an OH group 701 is combined with M₁, and anOH group 702 forms a coordinate bond so as to cross-link M₁ to M₂ (seeFIG. 6A).

Next, when energy of a given amount or more is applied to the oxidesemiconductor, H in the OH group 702 is dislocated to the OH group 701(see FIG. 6B) and an H₂O molecule 705 is generated. The H₂O molecule 705forms a coordinate bond with M₁ (see FIG. 6C). Finally, the H₂O molecule705 becomes an H₂O molecule 710 which is eliminated from M₁ to infinity(see FIG. 6D).

FIG. 6E shows values of potential energy of the states shown in FIGS. 6Ato 6D in the case where the combination of M₁ and M₂ is In and Ga.Energy 711 represents energy of the state shown in FIG. 6A. Energy 712represents energy of the state shown in FIG. 6B. Energy 713 representsenergy of the state shown in FIG. 6C. Energy 714 represents energy ofthe state shown in FIG. 6D.

The analysis results show that the activation energy for generating awater molecule in the case where the combination of M₁ and M₂ was In andGa was 1.14 eV. FIG. 6F shows calculation results of the activationenergy (Ea) for generating a water molecule in the case of the sixcombinations of M₁ and M₂. It was found that among the six combinationsof M₁ and M₂, the activation energy in the case of the combination of Inand Ga was lowest and the activation energy in the case of thecombination of Zn and Zn was highest. The activation energy tends toincrease when Zn is included in the combination of M₁ and M₂; therefore,Zn might be a hindrance to elimination of the OH group in theIn—Ga—Zn—O-based oxide semiconductor.

The analysis results show that the content (the number of atoms) of Inand the content (the number of atoms) of Ga are preferably substantiallythe same or the content of In is preferably larger than that of Ga forefficient elimination of the OH group in the In—Ga—Zn—O-based oxidesemiconductor by heat treatment. In addition, the analysis results showthat the content (the number of atoms) of Zn is preferably smaller thanthe sum of the content of In and the content of Ga and that Zn is morepreferably smaller than each content of In and Ga.

The composition of the oxide semiconductor is optimized, wherebydehydration or dehydrogenation by heat treatment can be performedefficiently.

Then, a conductive layer for forming a source and drain electrode layersis formed over the gate insulating layer 102 and the oxide semiconductorlayer 103.

The conductive layer for forming a source and drain electrode layers canbe formed using a metal conductive layer in a manner similar to that ofthe gate electrode layer 101. As the material of the metal conductivelayer, an element selected from Al, Cr, Cu, Ta, Ti, Mo, and W, an alloycontaining any of these elements as a component, an alloy containing anyof these elements in combination, or the like is preferably used. Forexample, a three-layer structure in which an aluminum layer is stackedover a titanium layer and a titanium layer is stacked over the aluminumlayer, or a three-layer structure in which an aluminum layer is stackedover a molybdenum layer and a molybdenum layer is stacked over thealuminum layer is preferable. Needless to say, the metal conductivelayer may have a single-layer structure, a two-layer structure, or astructure in which four or more layers are stacked.

In a photolithography process, with the use of a photomask, a sourceelectrode layer 105 a and a drain electrode layer 105 b are formed usingthe conductive layer for forming a source and drain electrode layers(see FIG. 1B). At this time, part of the oxide semiconductor layer 103is also etched, whereby the oxide semiconductor layer 103 having agroove (depression) is formed.

Note that a resist mask used for forming the source electrode layer 105a and the drain electrode layer 105 b may be formed by an inkjet method.Formation of the resist mask by an inkjet method needs no photomask,which results in a reduction in manufacturing costs.

An oxide conductive layer which has lower resistance than the oxidesemiconductor layer 103 may be formed between the oxide semiconductorlayer 103 and the source electrode layer 105 a and the drain electrodelayer 105 b. Such a stacked-layer structure makes it possible toincrease the withstand voltage of the thin film transistor.Specifically, the concentration of carriers in the oxide conductivelayer having lower resistance is preferably in the range of, forexample, 1×10²⁰/cm³ to 1×10²¹/cm³.

Then, an insulating layer 107 which covers the gate insulating layer102, the oxide semiconductor layer 103, the source electrode layer 105a, and the drain electrode layer 105 b and which is in contact with partof the oxide semiconductor layer 103 is formed (see FIG. 1C). Theinsulating layer 107 can be formed to a thickness of at least 1 nm ormore by a method by which impurities such as water and hydrogen areprevented from being mixed into the insulating layer 107, such as a CVDmethod or a sputtering method, as appropriate. Here, the insulatinglayer 107 is formed by, for example, a reactive sputtering method thatis a kind of sputtering method. The insulating layer 107 which is incontact with part of the oxide semiconductor layer 103 does not containimpurities such as moisture, hydrogen ions, and OW, and is formed usingan inorganic insulating layer which prevents entry of those impuritiesfrom the outside. The insulating layer 107 can be formed using,typically, a silicon oxide layer, a silicon nitride oxide layer, asilicon nitride layer, an aluminum oxide layer, or an aluminumoxynitride layer.

The insulating layer 107 may have a structure in which a silicon nitridelayer or an aluminum nitride layer is stacked over a silicon oxidelayer, a silicon nitride oxide layer, an aluminum oxide layer, or analuminum oxynitride layer. In particular, the silicon nitride layer ispreferable because it does not contain impurities such as moisture,hydrogen ions, or OW and prevents entry of these impurities from theoutside.

The substrate temperature at the time of the formation of the insulatinglayer 107 is preferably higher than or equal to room temperature andlower than or equal to 300° C. A silicon oxide layer can be formed by asputtering method in a rare gas (typically argon) atmosphere, an oxygenatmosphere, or an atmosphere containing a rare gas (typically argon) andoxygen. As a target, a silicon oxide target or a silicon target can beused. For example, silicon oxide can be deposited using a silicon targetin an atmosphere containing oxygen and a rare gas by a sputteringmethod.

Then, second heat treatment is performed. The second heat treatment isperformed at a temperature higher than or equal to 100° C. and lowerthan or equal to the highest temperature in the first heat treatment.For example, the substrate is put in an electric furnace which is a kindof heat treatment apparatus and heat treatment is performed in anitrogen atmosphere. The second heat treatment may be performed anytimeas long as it is performed after the formation of the insulating layer107.

Through the above-described steps, the channel-etched thin filmtransistor 150 can be manufactured in which the gate electrode layer 101is provided over the substrate 100 having an insulating surface, thegate insulating layer 102 is provided over the gate electrode layer 101,the oxide semiconductor layer 103 is provided over the gate insulatinglayer 102, the source electrode layer 105 a and the drain electrodelayer 105 b are provided over the oxide semiconductor layer 103, and theinsulating layer 107 which covers the gate insulating layer 102, theoxide semiconductor layer 103, the source electrode layer 105 a, and thedrain electrode layer 105 b and is in contact with part of the oxidesemiconductor layer 103 is provided.

FIG. 2 is a top view of the thin film transistor 150 described in thisembodiment. FIG. 1D illustrates a cross-sectional structure of a portiontaken along line X1-X2 in FIG. 2. In FIG. 2, L denotes channel length;W, channel width; A, the length of a region where the oxidesemiconductor layer 103 does not overlap with the source electrode layer105 a and the drain electrode layer 105 b in a direction parallel to achannel width direction; Ls, the length of a region where the sourceelectrode layer 105 a and the gate electrode layer 101 overlap with eachother; and Ld, the length of a region where the drain electrode layer105 b and the gate electrode layer 101 overlap with each other.

Although the thin film transistor 150 is described as a single-gate thinfilm transistor in this embodiment, the thin film transistor 150 can be,as needed, a multi-gate thin film transistor including a plurality ofchannel formation regions or a thin film transistor in which a secondgate electrode layer is provided over the insulating layer 107.

Further, although the method for manufacturing the channel-etched thinfilm transistor 150 is described in this embodiment, this embodiment isnot limited thereto. A bottom-gate thin film transistor (also referredto as an inverted coplanar thin film transistor) 160 illustrated in FIG.3A, a channel protective thin film transistor (also referred to as achannel stop thin film transistor) 170 including a channel protectivelayer 110 illustrated in FIG. 3B, or the like can be manufactured usinga material and a method which are similar to those of the channel-etchedthin film transistor 150. FIG. 3C illustrates another example of achannel-etched thin film transistor. A thin film transistor 180illustrated in FIG. 3C has a structure in which the outer side edges ofthe gate electrode layer 101 extend beyond the outer side edges of theoxide semiconductor layer 103.

Note that the channel length (L in FIG. 2) of the thin film transistoris defined by a distance between the source electrode layer 105 a andthe drain electrode layer 105 b, whereas the channel length of thechannel protective thin film transistor is defined by the width of thechannel protective layer in a direction parallel to a direction in whichcarriers flow.

In accordance with this embodiment, the threshold voltage of a thin filmtransistor in which an oxide semiconductor is used for a channelformation region can be made closer to 0 V.

Further, a thin film transistor including an oxide semiconductor layer,where the amount of change in threshold voltage before and after a BTtest performed under conditions of a processing temperature of 150° C.,a processing time of one hour, and an electric field intensity of 2×10⁶V/cm is less than or equal to 2 V, preferably less than or equal to 1.5V, more preferably less than or equal to 1 V can be manufactured.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

Embodiment 2

In this embodiment, an example will be described below in which thinfilm transistors which are placed in a pixel portion and at least someof driver circuits are formed over one substrate.

The thin film transistor placed in the pixel portion is formed inaccordance with Embodiment 1. Since the thin film transistor describedin Embodiment 1 is an n-channel TFT, some of driver circuits that can beconstituted by n-channel TFTs among the driver circuits are formed overa substrate over which the thin film transistor in the pixel portion isformed.

FIG. 7A illustrates an example of a block diagram of an active matrixdisplay device. A pixel portion 5301, a first scan line driver circuit5302, a second scan line driver circuit 5303, and a signal line drivercircuit 5304 are provided over a substrate 5300 in the display device.In the pixel portion 5301, a plurality of signal lines extended from thesignal line driver circuit 5304 are placed and a plurality of scan linesextended from the first scan line driver circuit 5302 and the secondscan line driver circuit 5303 are placed. Note that pixels eachincluding a display element are arranged in matrix in respective regionswhere the scan lines and the signal lines intersect with each other. Thesubstrate 5300 of the display device is connected to a timing controlcircuit 5305 (also referred to as a controller or a control IC) througha connection portion such as a flexible printed circuit (FPC).

In FIG. 7A, the first scan line driver circuit 5302, the second scanline driver circuit 5303, and the signal line driver circuit 5304 areformed over the substrate 5300 where the pixel portion 5301 is formed.Thus, the number of components of a driver circuit and the like that areexternally provided is reduced, which results in a reduction in costs.Moreover, the number of connections in the connection portion in thecase where wirings are extended from a driver circuit provided outsidethe substrate 5300 can be reduced, and the reliability or yield can beimproved.

Note that the timing control circuit 5305 supplies, for example, a firstscan line driver circuit start signal (GSP1) (also referred to as astart pulse) and a scan line driver circuit clock signal (GCK1) to thefirst scan line driver circuit 5302. Furthermore, the timing controlcircuit 5305 supplies, for example, a second scan line driver circuitstart signal (GSP2) and a scan line driver circuit clock signal (GCK2)to the second scan line driver circuit 5303. Moreover, the timingcontrol circuit 5305 supplies a signal line driver circuit start signal(SSP), a signal line driver circuit clock signal (SCK), video signaldata (DATA, also simply referred to as a video signal), and a latchsignal (LAT) to the signal line driver circuit 5304. Each clock signalmay be a plurality of clock signals with shifted phases or may besupplied together with a signal (CKB) obtained by inverting the clocksignal. Note that it is possible to omit one of the first scan linedriver circuit 5302 and the second scan line driver circuit 5303.

FIG. 7B illustrates a structure in which circuits with lower drivingfrequency (e.g., the first scan line driver circuit 5302 and the secondscan line driver circuit 5303) are formed over the substrate 5300 wherethe pixel portion 5301 is formed and the signal line driver circuit 5304is formed over a substrate which is different from the substrate 5300where the pixel portion 5301 is formed. With this structure, the drivercircuits formed over the substrate 5300 can be constituted by thin filmtransistors whose field effect mobility is lower than that oftransistors including a single crystal semiconductor. Thus, an increasein the size of the display device, a reduction in the number of steps, areduction in costs, an improvement in yield, or the like can beachieved.

The thin film transistor described in Embodiment 1 is an n-channel TFT.FIGS. 8A and 8B illustrate an example of a structure and operation of asignal line driver circuit constituted by n-channel TFTs.

The signal line driver circuit includes a shift register 5601 and aswitching circuit 5602. The switching circuit 5602 includes a pluralityof switching circuits 5602_1 to 5602_N (N is a natural number). Theswitching circuits 5602_1 to 5602_N each include a plurality of thinfilm transistors 5603_1 to 5603_k (k is a natural number). An example inwhich the thin film transistors 5603_1 to 5603_k are n-channel TFTs isdescribed below.

A connection relation in the signal line driver circuit is describedusing the switching circuit 5602_1 as an example. First terminals of thethin film transistors 5603_1 to 5603_k are connected to wirings 5604_1to 5604_k, respectively. Second terminals of the thin film transistors5603_1 to 5603_k are connected to signal lines S1 to Sk, respectively.Gates of the thin film transistors 5603_1 to 5603_k are connected to awiring 5605_1.

The shift register 5601 has a function of sequentially selecting theswitching circuits 5602_1 to 5602_N by sequentially outputting H-levelsignals (also referred to as H signals or signals at high power supplypotential level) to wirings 5605_1 to 5605_N.

The switching circuit 5602_1 has a function of controlling a conductionstate between the wirings 5604_1 to 5604_k and the signal lines S1 to Sk(electrical continuity between the first terminals and the secondterminals), that is, a function of controlling whether potentials of thewirings 5604_1 to 5604_k are supplied to the signal lines S1 to Sk. Inthis manner, the switching circuit 5602_1 functions as a selector.Moreover, the thin film transistors 5603_1 to 5603_k have a function ofcontrolling a conduction state between the wirings 5604_1 to 5604_k andthe signal lines S1 to Sk, respectively, that is, a function ofsupplying potentials of the wirings 5604_1 to 5604_k are supplied to thesignal lines S1 to Sk, respectively. In this manner, each of the thinfilm transistors 5603_1 to 5603_k functions as a switch.

The video signal data (DATA) is input to each of the wirings 5604_1 to5604_k. The video signal data (DATA) is often an analog signalcorresponding to an image signal or image data.

Next, the operation of the signal line driver circuit in FIG. 8A will bedescribed with reference to a timing chart of FIG. 8B. FIG. 8Billustrates examples of signals Sout_1 to Sout_N and signals Vdata_1 toVdata_k. The signals Sout_1 to Sout_N are examples of output signalsfrom the shift register 5601. The signals Vdata_1 to Vdata_k areexamples of signals input to the wirings 5604_1 to 5604_k. Note that oneoperation period of the signal line driver circuit corresponds to onegate selection period in a display device. For example, one gateselection period is divided into periods T1 to TN. Each of the periodsT1 to TN is a period for writing the video signal data (DATA) into apixel in a selected row.

Note that signal waveform distortion and the like in each structureillustrated in drawings and the like in this embodiment are exaggeratedfor simplicity in some cases. Therefore, this embodiment is notnecessarily limited to the scale illustrated in the drawing and thelike.

In the periods T1 to TN, the shift register 5601 sequentially outputs anH-level signals to the wirings 5605_1 to 5605_N. For example, in theperiod T1, the shift register 5601 outputs an H-level signal to thewiring 5605_1. Then, the thin film transistors 5603_1 to 5603_k areturned on, so that the wirings 5604_1 to 5604_k and the signal lines S1to Sk are brought into conduction. At this time, Data(S1) to Data(Sk)are input to the wirings 5604_1 to 5604_k, respectively. The Data(S1) toData(Sk) are written into pixels in a first to kth columns in a selectedrow through the thin film transistors 5603_1 to 5603_k, respectively. Insuch a manner, in the periods T1 to TN, the video signal data (DATA) aresequentially written into the pixels in the selected row by k columns.

The video signal data (DATA) are written into pixels by a plurality ofcolumns as described above, whereby the number of video signal data(DATA) or the number of wirings can be reduced. Consequently, the numberof connections with an external circuit can be reduced. Moreover, thetime for writing can be extended when video signals are written intopixels by a plurality of columns; thus, insufficient writing of videosignals can be prevented.

Note that the circuit constituted by the thin film transistor inEmbodiment 1 can be used for the shift register 5601 and the switchingcircuit 5602. In that case, the shift register 5601 can be constitutedby only n-channel transistors or only p-channel transistors.

One embodiment of a shift register which is used for part of the scanline driver circuit and/or the signal line driver circuit will bedescribed with reference to FIGS. 9A to 9D and FIGS. 10A and 10B.

The scan line driver circuit includes a shift register. The scan linedriver circuit may additionally include a level shifter, a buffer, orthe like in some cases. In the scan line driver circuit, a clock signal(CK) and a start pulse signal (SP) are input to the shift register, sothat a selection signal is generated. The selection signal generated isbuffered and amplified by the buffer, and the resulting signal issupplied to a corresponding scan line. Gate electrodes of transistors inpixels of one line are connected to a scan line. Since the transistorsin the pixels of one line have to be turned on all at once, a bufferthat can supply a large current is used.

The shift register includes a first to Nth pulse output circuits 10_1 to10_N (N is a natural number greater than or equal to 3) (see FIG. 9A).In the shift register illustrated in FIG. 9A, a first clock signal CK1,a second clock signal CK2, a third clock signal CK3, and a fourth clocksignal CK4 are supplied from a first wiring 11, a second wiring 12, athird wiring 13, and a fourth wiring 14, respectively, to the first toNth pulse output circuits 10_1 to 10_N. A start pulse SP1 (a first startpulse) is input from a fifth wiring 15 to the first pulse output circuit10_1. To the nth pulse output circuit 10_n (n is a natural numbergreater than or equal to 2 and less than or equal to N) in the second orlater stage, a signal from the pulse output circuit in the precedingstage (such a signal is referred to as a preceding-stage signalOUT(n−1)) (n is a natural number greater than or equal to 2) is input. Asignal from the third pulse output circuit 10_3 in the stage that is twostages after the first pulse output circuit 10_1 is also input to thefirst pulse output circuit 10_1. In a similar manner, a signal from the(n+2)th pulse output circuit 10_(n+2) in the stage that is two stagesafter the nth pulse output circuit 10_n (such a signal is referred to asa later-stage signal OUT(n+2)) is input to the nth pulse output circuit10_n in the second or later stage. Thus, the pulse output circuits inthe respective stages output first output signals (OUT(1)(SR) to OUT(N)(SR)) to be input to the pulse output circuits in the respectivesubsequent stages and/or the pulse output circuits in the stages thatare two stages before the respective pulse output circuits and secondoutput signals (OUT(1) to OUT (N)) to be input to other circuits or thelike. Note that as illustrated in FIG. 9A, since the later-stage signalOUT(n+2) is not input to the pulse output circuits in the last twostages of the shift register, for example, a second start pulse SP2 anda third start pulse SP3 may be additionally input to the respectivepulse output circuits.

Note that a clock signal (CK) is a signal that alternates between an Hlevel and an L level (also referred to as an L signal or a signal at lowpower supply potential level) at regular intervals. Here, the firstclock signal (CK1) to the fourth clock signal (CK4) are delayed by ¼cycle sequentially (i.e., they are 90° out of phase with each other). Inthis embodiment, driving of the pulse output circuit is controlled withthe first to fourth clock signals (CK1) to (CK4). Note that the clocksignal is also referred to as GCK or SCK in some cases depending on adriver circuit to which the clock signal is input; the clock signal isreferred to as CK in the following description.

A first input terminal 21, a second input terminal 22, and a third inputterminal 23 are electrically connected to any of the first to fourthwirings 11 to 14. For example, in the first pulse output circuit 10_1 inFIG. 9A, the first input terminal 21 is electrically connected to thefirst wiring 11, the second input terminal 22 is electrically connectedto the second wiring 12, and the third input terminal 23 is electricallyconnected to the third wiring 13. In the second pulse output circuit10_2, the first input terminal 21 is electrically connected to thesecond wiring 12, the second input terminal 22 is electrically connectedto the third wiring 13, and the third input terminal 23 is electricallyconnected to the fourth wiring 14.

Each of the first to Nth pulse output circuits 10_1 to 10_N includes thefirst input terminal 21, the second input terminal 22, the third inputterminal 23, a fourth input terminal 24, a fifth input terminal 25, afirst output terminal 26, and a second output terminal 27 (see FIG. 9B).In the first pulse output circuit 10_1, the first clock signal CK1 isinput to the first input terminal 21; the second clock signal CK2 isinput to the second input terminal 22; the third clock signal CK3 isinput to the third input terminal 23; a start pulse is input to thefourth input terminal 24; a later-stage signal OUT(3) is input to thefifth input terminal 25; the first output signal OUT(1)(SR) is outputfrom the first output terminal 26; and the second output signal OUT(1)is output from the second output terminal 27.

In the first to Nth pulse output circuits 10_1 to 10_N, a thin filmtransistor having four terminals can be used as well as a thin filmtransistor having three terminals. The thin film transistor having fourterminals includes a source electrode, a drain electrode, a first gateelectrode, and a second gate electrode, where a channel formation regionof an oxide semiconductor layer is provided between the first and secondgate electrodes with insulating layers interposed between the first andsecond gate electrodes. FIG. 9C illustrates the symbol of a thin filmtransistor 28 having four terminals and the symbol is used in drawingsor the like below. The thin film transistor 28 can control electriccurrent between an IN terminal and an OUT terminal with a first controlsignal G1 which is input to a first gate electrode and a second controlsignal G2 which is input to a second gate electrode.

Further, the threshold voltage of the thin film transistor 28illustrated in FIG. 9C can be controlled to be a desired level bycontrol of the potential of the first gate electrode or the potential ofthe second gate electrode.

Next, an example of a specific circuit configuration of the pulse outputcircuit will be described with reference to FIG. 9D.

The first pulse output circuit 10_1 includes first to thirteenthtransistors 31 to 43 (see FIG. 9D). A signal or a power supply potentialis supplied to the first to thirteenth transistors 31 to 43 from a powersupply line 51 to which a first high power supply potential VDD issupplied, a power supply line 52 to which a second high power supplypotential VCC is supplied, and a power supply line 53 to which a lowpower supply potential VSS is supplied, in addition to the first tofifth input terminals 21 to 25, the first output terminal 26, and thesecond output terminal 27, which are described above. The relation ofthe power supply potentials of the power supply lines in FIG. 9D is asfollows: the first power supply potential VDD is higher than or equal tothe second power supply potential VCC, and the second power supplypotential VCC is higher than the third power supply potential VSS. Notethat the first to fourth clock signals (CK1) to (CK4) alternate betweenan H level and an L level at regular intervals; the clock signal at theH level is VDD and the clock signal at the L level is VSS. By making thepotential VDD of the power supply line 51 higher than the potential VCCof the power supply line 52, a potential applied to a gate electrode ofa transistor can be lowered, shift in the threshold voltage of thetransistor can be reduced, and degradation of the transistor can besuppressed without an adverse effect on the operation of the transistor.Note that as illustrated in FIG. 9D, the thin film transistor 28 havingfour terminals that is illustrated in FIG. 9C is preferably used as thefirst transistor 31 and the sixth to ninth transistors 36 to 39 amongthe first to thirteenth transistors 31 to 43. The first transistor 31and the sixth to ninth transistors 36 to 39 need to operate so that apotential of a node to which one electrode serving as a source or adrain is connected is switched with a control signal of the gateelectrode, and can further reduce a malfunction of the pulse outputcircuit because response to the control signal input to the gateelectrode is fast (the rise of on-state current is steep). Thus, withthe use of the thin film transistor 28 having four terminals which isillustrated in FIG. 9C, the threshold voltage can be controlled, and amalfunction of the pulse output circuit can be further prevented. Notethat although the first control signal G1 and the second control signalG2 are the same control signals in FIG. 9D, the first control signal G1and the second control signal G2 may be different control signals.

In FIG. 9D, a first terminal of the first transistor 31 is electricallyconnected to the power supply line 51, a second terminal of the firsttransistor 31 is electrically connected to a first terminal of the ninthtransistor 39, and gate electrodes (a first gate electrode and a secondgate electrode) of the first transistor 31 are electrically connected tothe fourth input terminal 24. A first terminal of the second transistor32 is electrically connected to the power supply line 53, a secondterminal of the second transistor 32 is electrically connected to thefirst terminal of the ninth transistor 39, and a gate electrode of thesecond transistor 32 is electrically connected to a gate electrode ofthe fourth transistor 34. A first terminal of the third transistor 33 iselectrically connected to the first input terminal 21, and a secondterminal of the third transistor 33 is electrically connected to thefirst output terminal 26. A first terminal of the fourth transistor 34is electrically connected to the power supply line 53, and a secondterminal of the fourth transistor 34 is electrically connected to thefirst output terminal 26. A first terminal of the fifth transistor 35 iselectrically connected to the power supply line 53, a second terminal ofthe fifth transistor 35 is electrically connected to the gate electrodeof the second transistor 32 and the gate electrode of the fourthtransistor 34, and a gate electrode of the fifth transistor 35 iselectrically connected to the fourth input terminal 24. A first terminalof the sixth transistor 36 is electrically connected to the power supplyline 52, a second terminal of the sixth transistor 36 is electricallyconnected to the gate electrode of the second transistor 32 and the gateelectrode of the fourth transistor 34, and gate electrodes (a first gateelectrode and a second gate electrode) of the sixth transistor 36 areelectrically connected to the fifth input terminal 25. A first terminalof the seventh transistor 37 is electrically connected to the powersupply line 52, a second terminal of the seventh transistor 37 iselectrically connected to a second terminal of the eighth transistor 38,and gate electrodes (a first gate electrode and a second gate electrode)of the seventh transistor 37 are electrically connected to the thirdinput terminal 23. A first terminal of the eighth transistor 38 iselectrically connected to the gate electrode of the second transistor 32and the gate electrode of the fourth transistor 34, and gate electrodes(a first gate electrode and a second gate electrode) of the eighthtransistor 38 are electrically connected to the second input terminal22. The first terminal of the ninth transistor 39 is electricallyconnected to the second terminal of the first transistor 31 and thesecond terminal of the second transistor 32, a second terminal of theninth transistor 39 is electrically connected to the gate electrode ofthe third transistor 33 and a gate electrode of the tenth transistor 40,and gate electrodes (a first gate electrode and a second gate electrode)of the ninth transistor 39 are electrically connected to the powersupply line 52. A first terminal of the tenth transistor 40 iselectrically connected to the first input terminal 21, a second terminalof the tenth transistor 40 is electrically connected to the secondoutput terminal 27, and the gate electrode of the tenth transistor 40 iselectrically connected to the second terminal of the ninth transistor39. A first terminal of the eleventh transistor 41 is electricallyconnected to the power supply line 53, a second terminal of the eleventhtransistor 41 is electrically connected to the second output terminal27, and a gate electrode of the eleventh transistor 41 is electricallyconnected to the gate electrode of the second transistor 32 and the gateelectrode of the fourth transistor 34. A first terminal of the twelfthtransistor 42 is electrically connected to the power supply line 53, asecond terminal of the twelfth transistor 42 is electrically connectedto the second output terminal 27, and a gate electrode of the twelfthtransistor 42 is electrically connected to the gate electrodes (thefirst gate electrode and the second gate electrode) of the seventhtransistor 37. A first terminal of the thirteenth transistor 43 iselectrically connected to the power supply line 53, a second terminal ofthe thirteenth transistor 43 is electrically connected to the firstoutput terminal 26, and a gate electrode of the thirteenth transistor 43is electrically connected to the gate electrodes (the first gateelectrode and the second gate electrode) of the seventh transistor 37.

In FIG. 9D, a portion where the gate electrode of the third transistor33, the gate electrode of the tenth transistor 40, and the secondterminal of the ninth transistor 39 are connected is referred to as anode A. Moreover, a portion where the gate electrode of the secondtransistor 32, the gate electrode of the fourth transistor 34, thesecond terminal of the fifth transistor 35, the second terminal of thesixth transistor 36, the first terminal of the eighth transistor 38, andthe gate electrode of the eleventh transistor 41 are connected isreferred to as a node B.

Note that a thin film transistor is an element having at least threeterminals of a gate, a drain, and a source. The thin film transistor hasa channel region between a drain region and a source region, and currentcan flow through the drain region, the channel region, and the sourceregion. Here, since the source and the drain of the thin film transistormay change depending on the structure, the operating condition, and thelike of the thin film transistor, it is difficult to define which is asource or a drain. Therefore, a region functioning as the source or thedrain is not called the source or the drain in some cases. In that case,for example, such regions may be referred to as a first terminal and asecond terminal, respectively.

Further, functions of a source and a drain might be switched whentransistors having different polarities are employed or a direction ofcurrent flow is changed in circuit operation, for example. Therefore,the terms “source” and “drain” can be switched in this specification.

Note that in FIG. 9D and FIG. 10A, a capacitor for performing bootstrapoperation by bringing the node A into a floating state may beadditionally provided. Furthermore, a capacitor having one electrodeelectrically connected to the node B may be additionally provided inorder to hold a potential of the node B.

FIG. 10B is a timing chart of a shift register including a plurality ofpulse output circuits illustrated in FIG. 10A. Note that when the shiftregister is included in a scan line driver circuit, a period 61 in FIG.10B corresponds to a vertical retrace period and a period 62 correspondsto a gate selection period.

Note that the placement of the ninth transistor 39 in which the secondpower supply potential VCC is applied to the gate as illustrated in FIG.10A has the following advantages before and after bootstrap operation.

Without the ninth transistor 39 in which the second power supplypotential VCC is applied to the gate electrode, if a potential of thenode A is raised by bootstrap operation, a potential of the source whichis the second terminal of the first transistor 31 rises to a valuehigher than the first power supply potential VDD. Then, the source ofthe first transistor 31 is switched to the first terminal, that is, theterminal on the power supply line 51 side. Consequently, in the firsttransistor 31, a high bias voltage is applied and thus significantstress is applied between the gate and the source and between the gateand the drain, which might cause deterioration of the transistor. On theother hand, with the ninth transistor 39 in which the second powersupply potential VCC is applied to the gate electrode, increase in thepotential of the second terminal of the first transistor 31 can beprevented while the potential of the node A is raised by bootstrapoperation. In other words, the placement of the ninth transistor 39makes it possible to lower the level of the negative bias voltageapplied between the gate and the source of the first transistor 31.Thus, the circuit configuration in this embodiment allows a negativebias voltage applied between the gate and the source of the firsttransistor 31 to be reduced, whereby deterioration of the firsttransistor 31 due to stress can be prevented.

Note that the ninth transistor 39 can be provided anywhere as long asthe first terminal and the second terminal of the ninth transistor 39are connected between the second terminal of the first transistor 31 andthe gate of the third transistor 33. Note that in the case where theshift register including a plurality of pulse output circuits in thisembodiment is included in a signal line driver circuit having a largernumber of stages than a scan line driver circuit, the ninth transistor39 can be omitted, which results in a reduction in the number oftransistors.

Note that an oxide semiconductor is used for a semiconductor layer ineach of the first to thirteenth transistors 31 to 43, and thus theoff-state current of the thin film transistors can be reduced, theon-state current and field effect mobility can be increased, and thedegree of degradation of the transistors can be reduced. As a result, amalfunction in the circuit can be prevented. Moreover, the degree ofdeterioration of the transistor using an oxide semiconductor byapplication of a high potential to a gate electrode is smaller than thatof a transistor using amorphous silicon. Consequently, similar operationcan be obtained even when the first power supply potential VDD issupplied to the power supply line to which the second power supplypotential VCC is supplied, and the number of power supply lines placedbetween circuits can be reduced;

thus, the size of the circuit can be reduced.

Note that a similar function is obtained even when the connectionrelation is changed so that a clock signal that is supplied to the gateelectrodes (the first gate electrode and the second gate electrode) ofthe seventh transistor 37 from the third input terminal 23 and a clocksignal that is supplied to the gate electrodes (the first gate electrodeand the second gate electrode) of the eighth transistor 38 from thesecond input terminal 22 are supplied from the second input terminal 22and the third input terminal 23, respectively. In the shift registerillustrated in FIG. 10A, a state of the seventh transistor 37 and theeighth transistor 38 is changed so that both the seventh transistor 37and the eighth transistor 38 are on, then the seventh transistor 37 isoff and the eighth transistor 38 is on, and then the seventh transistor37 and the eighth transistor 38 are off; thus, the fall in potential ofthe node B due to fall in the potentials of the second input terminal 22and the third input terminal 23 is caused twice by fall in the potentialof the gate electrode of the seventh transistor 37 and fall in thepotential of the gate electrode of the eighth transistor 38. On theother hand, in the shift register illustrated in FIG. 10A, when a stateof the seventh transistor 37 and the eighth transistor 38 is changed asin the period in FIG. 10B so that both the seventh transistor 37 and theeighth transistor 38 are on, then the seventh transistor 37 is on andthe eighth transistor 38 is off, and then the seventh transistor 37 andthe eighth transistor 38 are off, the fall in potential of the node Bdue to fall in potentials of the second input terminal 22 and the thirdinput terminal 23 is reduced to one, which is caused by fall inpotential of the gate electrode of the eighth transistor 38.Consequently, connection relation where the clock signal CK3 is suppliedfrom the third input terminal 23 to the gate electrodes (the first gateelectrode and the second gate electrode) of the seventh transistor 37and the clock signal CK2 is supplied from the second input terminal 22to the gate electrodes (the first gate electrode and the second gateelectrode) of the eighth transistor 38 is preferably used, in which casethe number of change in the potential of the node B can be reduced andnoise can be reduced.

In such a manner, an H-level signal is regularly supplied to the node Bin a period during which the potentials of the first output terminal 26and the second output terminal 27 are held at L level; thus, amalfunction of the pulse output circuit can be prevented.

The thin film transistor in the driver circuit is manufactured by themanufacturing method of the thin film transistor which is described inEmbodiment 1, whereby high speed operation of the thin film transistorin the driver circuit portion can be realized and power saving can beachieved.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

Embodiment 3

In this embodiment, a case will be described where thin film transistorsare manufactured and a semiconductor device having a display function(also referred to as a display device) in which the thin filmtransistors are used in a pixel portion and driver circuits ismanufactured. Further, some or all of the driver circuits including thethin film transistors can be formed over a substrate where the pixelportion is formed, whereby a system-on-panel can be obtained.

The display device includes a display element. As the display element, aliquid crystal element (also referred to as a liquid crystal displayelement) or a light-emitting element (also referred to as alight-emitting display element) can be used.

The light-emitting element includes, in its category, an element whoseluminance is controlled by current or voltage, and specifically includesan inorganic electroluminescent (EL) element, an organic EL element, andthe like. Furthermore, a display medium whose contrast is changed by anelectric effect, such as electronic ink, can be used.

In addition, the display device includes a panel in which the displayelement is sealed, and a module in which an IC or the like including acontroller is mounted on the panel. The display device relates to onemode of an element substrate before the display element is completed ina manufacturing process of the display device, and the element substrateis provided with a means for supplying current to the display element ineach of a plurality of pixels. Specifically, the element substrate maybe in a state in which only a pixel electrode of the display element, astate in which a conductive layer to be a pixel electrode is formed butis not etched yet to form the pixel electrode, or any other states.

Note that a display device in this specification refers to an imagedisplay device, a display device, or a light source (including alighting device). Further, the display device includes the followingmodules in its category: a module including a connector such as aflexible printed circuit (FPC), a tape automated bonding (TAB) tape, ora tape carrier package (TCP); a module having a TAB tape or a TCP thatis provided with a printed wiring board at the end thereof; and a modulehaving an integrated circuit (IC) that is directly mounted on a displayelement by a chip on glass (COG) method.

In this embodiment, an example of a liquid crystal display device willbe described as a semiconductor device which is one embodiment of thepresent invention. The appearance and a cross section of a liquidcrystal display panel, which is one embodiment of a semiconductordevice, are described with reference to FIGS. 11A1, 11A2, and 11B. FIGS.11A1 and 11A2 are each a top view of a panel in which highly reliablethin film transistors 4010 and 4011 which include a semiconductor layerof an In—Ga—Zn—O-based non-single-crystal layer, and a liquid crystalelement 4013, which are formed over a first substrate 4001, are sealedbetween the first substrate 4001 and a second substrate 4006 with asealant 4005. FIG. 11B corresponds to a cross-sectional view of FIGS.11A1 and 11A2 along line M-N.

The sealant 4005 is provided so as to surround a pixel portion 4002 anda scan line driver circuit 4004 which are provided over the firstsubstrate 4001. The second substrate 4006 is provided over the pixelportion 4002 and the scan line driver circuit 4004. Consequently, thepixel portion 4002 and the scan line driver circuit 4004 are sealedtogether with a liquid crystal layer 4008, by the first substrate 4001,the sealant 4005, and the second substrate 4006. A signal line drivercircuit 4003 that is formed using a single crystal semiconductor or apolycrystalline semiconductor over a substrate separately prepared ismounted in a region that is different from the region surrounded by thesealant 4005 over the first substrate 4001.

Note that there is no particular limitation on the connection method ofthe driver circuit which is separately formed, and a COG method, a wirebonding method, a TAB method, or the like can be used. FIG. 11A1illustrates an example in which the signal line driver circuit 4003 ismounted by a COG method. FIG. 11A2 illustrates an example in which thesignal line driver circuit 4003 is mounted by a TAB method.

The pixel portion 4002 and the scan line driver circuit 4004 providedover the first substrate 4001 each include a plurality of thin filmtransistors. FIG. 11B illustrates, as an example, the thin filmtransistor 4010 included in the pixel portion 4002 and the thin filmtransistor 4011 included in the scan line driver circuit 4004.Insulating layers 4020 and 4021 are provided over the thin filmtransistors 4010 and 4011.

Any of the highly reliable thin film transistors including the oxidesemiconductor layers which are described in Embodiment 1 can be used asthe thin film transistors 4010 and 4011. In this embodiment, the thinfilm transistors 4010 and 4011 are n-channel thin film transistors.

A conductive layer 4040 is provided over part of the insulating layer4021, which overlaps with a channel formation region of an oxidesemiconductor layer in the thin film transistor 4011 for the drivercircuit. The conductive layer 4040 is provided at the positionoverlapping with the channel formation region of the oxide semiconductorlayer, whereby the amount of change in the threshold voltage of the thinfilm transistor 4011 before and after the BT test can be reduced. Thepotential of the conductive layer 4040 may be the same or different fromthat of a gate electrode layer of the thin film transistor 4011. Theconductive layer 4040 can also function as a second gate electrodelayer. Alternatively, the potential of the conductive layer 4040 may beGND or 0 V, or the conductive layer 4040 may be in a floating state.

A pixel electrode layer 4030 included in the liquid crystal element 4013is electrically connected to the thin film transistor 4010. A counterelectrode layer 4031 of the liquid crystal element 4013 is formed on thesecond substrate 4006. A portion where the pixel electrode layer 4030,the counter electrode layer 4031, and the liquid crystal layer 4008overlap with one another corresponds to the liquid crystal element 4013.Note that the pixel electrode layer 4030 and the counter electrode layer4031 are provided with an insulating layer 4032 and an insulating layer4033 functioning as alignment films, respectively, and the liquidcrystal layer 4008 is sandwiched between the pixel electrode layer 4030and the counter electrode layer 4031 with the insulating layers 4032 and4033 therebetween.

Note that the first substrate 4001 and the second substrate 4006 can beformed of glass, metal (typically, stainless steel), ceramic, orplastic. As plastic, a fiberglass-reinforced plastics (FRP) plate, apolyvinyl fluoride (PVF) film, a polyester film, or an acrylic resinfilm can be used. In addition, a sheet with a structure in which analuminum foil is sandwiched between PVF films or polyester films can beused.

A columnar spacer denoted by reference numeral 4035 is obtained byselective etching of an insulating film and is provided in order tocontrol the distance (a cell gap) between the pixel electrode layer 4030and the counter electrode layer 4031. Alternatively, a spherical spacermay be used. The counter electrode layer 4031 is electrically connectedto a common potential line formed over the substrate where the thin filmtransistor 4010 is formed. The counter electrode layer 4031 and thecommon potential line can be electrically connected to each otherthrough conductive particles arranged between the pair of substratesusing the common connection portion. Note that the conductive particlesare included in the sealant 4005.

In addition, liquid crystals exhibiting a blue phase for which analignment film is unnecessary may be used. The blue phase is one ofliquid crystal phases, which appears just before a cholesteric phasechanges into an isotropic phase while the temperature of a cholestericliquid crystal is increased. Since the blue phase only appears within anarrow range of temperature, the liquid crystal layer 4008 is formedusing a liquid crystal composition in which a chiral agent is mixed at 5wt % or more in order to increase the temperature range. The liquidcrystal composition including liquid crystals exhibiting a blue phaseand a chiral agent has a short response time of 1 msec or less and isoptically isotropic; thus, alignment treatment is not needed and viewingangle dependence is small.

Note that the liquid crystal display device described in this embodimentis an example of a transmissive liquid crystal display device; however,the liquid crystal display device can be applied to either a reflectiveliquid crystal display device or a semi-transmissive liquid crystaldisplay device.

An example of the liquid crystal display device described in thisembodiment is illustrated in which a polarizing plate is provided on theouter surface of the substrate (on the viewer side) and a coloring layerand an electrode layer used for a display element are provided on theinner surface of the substrate in that order; however, the polarizingplate may be provided on the inner surface of the substrate. The layeredstructure of the polarizing plate and the coloring layer is not limitedto that in this embodiment and may be set as appropriate depending onmaterials of the polarizing plate and the coloring layer or theconditions of the manufacturing process. A light-blocking layer whichfunctions as a black matrix may be provided when needed.

In this embodiment, in order to reduce surface unevenness of the thinfilm transistors and to improve reliability of the thin filmtransistors, the thin film transistors are covered with a protectivelayer or the insulating layers (the insulating layer 4020 and theinsulating layer 4021) which function as planarization insulatinglayers. Note that the protective layer is provided to prevent entry of acontaminant impurity such as an organic substance, a metal substance, ormoisture floating in the air and is preferably a dense film. Theprotective layer may be formed with a single layer or a stacked layer ofa silicon oxide layer, a silicon nitride layer, a silicon oxynitridelayer, a silicon nitride oxide layer, an aluminum oxide layer, analuminum nitride layer, aluminum oxynitride layer, and/or an aluminumnitride oxide layer by a sputtering method. In this embodiment, anexample in which the protective layer is formed by a sputtering methodis described; however, there is no particular limitation on a method,and various kinds of methods may be used.

Here, the insulating layer 4020 having a stacked-layer structure isformed as the protective layer. Here, as a first layer of the insulatinglayer 4020, a silicon oxide layer is formed by a sputtering method. Theuse of a silicon oxide layer for the protective layer provides anadvantageous effect of preventing hillock of an aluminum layer used fora source electrode layer and a drain electrode layer.

An insulating layer is formed as a second layer of the protective layer.Here, as a second layer of the insulating layer 4020, a silicon nitridelayer is formed by a sputtering method. The use of the silicon nitridelayer as the protective layer can prevent mobile ions such as sodiumions from entering a semiconductor region, thereby suppressingvariations in electric characteristics of the TFT.

After the protective layer is formed, annealing (300° C. to 400° C.) ofthe semiconductor layer may be performed.

The insulating layer 4021 is formed as the planarization insulatinglayer. As the insulating layer 4021, an organic material having heatresistance, such as polyimide, an acrylic resin, abenzocyclobutene-based resin, polyamide, or an epoxy resin, can be used.Besides the above organic materials, it is also possible to use alow-dielectric constant material (a low-k material), a siloxane-basedresin, PSG (phosphosilicate glass), BPSG (borophosphosilicate glass), orthe like. Note that the insulating layer 4021 may be formed by stackinga plurality of insulating layers formed of these materials.

Note that the siloxane-based resin corresponds to a resin including aSi—O—Si bond formed using a siloxane-based material as a startingmaterial. The siloxane-based resin may include as a substituent anorganic group (e.g., an alkyl group or an aryl group) or a fluoro group.In addition, the organic group may include a fluoro group.

There is no particular limitation on the method of forming theinsulating layer 4021. The insulating layer 4021 can be formed,depending on the material, by a method such as sputtering, an SOGmethod, a spin coating method, a dipping method, a spray coating method,a droplet discharge method (e.g., an inkjet method, screen printing, oroffset printing), or a tool such as a doctor knife, a roll coater, acurtain coater, or a knife coater. In the case where the insulatinglayer 4021 is formed using a material solution, annealing (300° C. to400° C.) of the semiconductor layer may be performed at the same time asa baking step. The baking step of the insulating layer 4021 also servesas annealing of the semiconductor layer, whereby a semiconductor devicecan be manufactured efficiently.

The pixel electrode layer 4030 and the counter electrode layer 4031 canbe formed using a light-transmitting conductive material such as indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium tin oxide (hereinafter referred to asITO), indium zinc oxide, indium tin oxide to which silicon oxide isadded, or the like.

Conductive compositions including a conductive high molecule (alsoreferred to as a conductive polymer) can be used for the pixel electrodelayer 4030 and the counter electrode layer 4031. The pixel electrodeformed using the conductive composition preferably has a sheetresistance of less than or equal to 10000 ohms per square and atransmittance of greater than or equal to 70% at a wavelength of 550 nm.Further, the resistivity of the conductive high molecule included in theconductive composition is preferably less than or equal to 0.1 Ω·cm.

As the conductive high molecule, a so-called π-electron conjugatedconductive polymer can be used. As examples thereof, polyaniline or aderivative thereof, polypyrrole or a derivative thereof, polythiopheneor a derivative thereof, a copolymer of two or more kinds of them, andthe like can be given.

Further, a variety of signals and potentials are supplied to the signalline driver circuit 4003 which is formed separately, the scan linedriver circuit 4004, or the pixel portion 4002 from an FPC 4018.

In this embodiment, a connection terminal electrode 4015 is formed usingthe same conductive layer as the pixel electrode layer 4030 included inthe liquid crystal element 4013. A terminal electrode 4016 is formedusing the same conductive layer as the source and drain electrode layersincluded in the thin film transistors 4010 and 4011.

The connection terminal electrode 4015 is electrically connected to aterminal included in the FPC 4018 through an anisotropic conductivelayer 4019.

FIGS. 11A1, 11A2, and 11B illustrate an example in which the signal linedriver circuit 4003 is formed separately and mounted on the firstsubstrate 4001; however, this embodiment is not limited to thisstructure. The scan line driver circuit may be separately formed andthen mounted, or only part of the signal line driver circuit or part ofthe scan line driver circuit may be separately formed and then mounted.

FIG. 12 illustrates an example in which a liquid crystal display modulewhich corresponds to one embodiment of a semiconductor device is formedusing a TFT substrate 2600.

FIG. 12 illustrates an example of the liquid crystal display module, inwhich the TFT substrate 2600 and a counter substrate 2601 are fixed toeach other with a sealant 2602, and a pixel portion 2603 including a TFTand the like, a display element 2604 including a liquid crystal layer,and a coloring layer 2605 are provided between the substrates to form adisplay region. The coloring layer 2605 is necessary to perform colordisplay. In the RGB system, coloring layers corresponding to colors ofred, green, and blue are provided for pixels. Polarizing plates 2606 and2607 and a diffusion plate 2613 are provided outside the TFT substrate2600 and the counter substrate 2601. A light source includes a coldcathode tube 2610 and a reflection plate 2611. A circuit board 2612 isconnected to a wiring circuit portion 2608 of the TFT substrate 2600 bya flexible wiring board 2609 and includes an external circuit such as acontrol circuit or a power source circuit. The polarizing plate and theliquid crystal layer may be stacked with a retardation platetherebetween.

For the liquid crystal display module, a twisted nematic (TN) mode, anin-plane-switching (IPS) mode, a fringe field switching (FFS) mode, amulti-domain vertical alignment (MVA) mode, a patterned verticalalignment (PVA) mode, an axially symmetric aligned micro-cell (ASM)mode, an optically compensated birefringence (OCB) mode, a ferroelectricliquid crystal (FLC) mode, an antiferroelectric liquid crystal (AFLC)mode, or the like can be used.

Through the above-described steps, a highly reliable liquid crystaldisplay device can be manufactured as a semiconductor device.

Thin film transistors in the pixel portion of the liquid crystal displaydevice are manufactured using the thin film transistor described inEmbodiment 1, whereby display unevenness caused by variation inthreshold voltage of the thin film transistors in pixels can besuppressed.

Further, the thin film transistors in the driver circuit portion of theliquid crystal display device are manufactured by the manufacturingmethod of the thin film transistor which is described in Embodiment 1,whereby high-speed operation of the thin film transistors in the drivercircuit portion can be realized and power saving can be achieved.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

Embodiment 4

In this embodiment, an example of electronic paper will be described asone embodiment of a semiconductor device.

The thin film transistor described in Embodiment 1 can be used forelectronic paper in which electronic ink is driven by an elementelectrically connected to a switching element. The electronic paper isalso referred to as an electrophoretic display device (anelectrophoretic display) and has advantages such as the same level ofreadability as regular paper, lower power consumption than other displaydevices, thinness, and lightness in weight.

Electrophoretic displays can have various modes. Electrophoreticdisplays contain a plurality of microcapsules dispersed in a solvent ora solute, each of which contains first particles that are positivelycharged and second particles that are negatively charged. By applicationof an electric field to the microcapsules, the particles in themicrocapsules move in opposite directions to each other and only thecolor of the particles gathering on one side is displayed. Note that thefirst particles and the second particles each include a pigment and donot move without an electric field. Moreover, the first particles andthe second particles have different colors (which may be colorless).

Thus, an electrophoretic display utilizes a so-called dielectrophoreticeffect by which a substance having a high dielectric constant moves to ahigh-electric field region.

A solution in which the above-described microcapsules are dispersed in asolvent is referred to as electronic ink. This electronic ink can beprinted on a surface of glass, plastic, cloth, paper, or the like.Furthermore, color display is possible with a color filter or particlesincluding a pigment.

When a plurality of the above-described microcapsules are arranged asappropriate over an active matrix substrate so as to be sandwichedbetween two electrodes, an active matrix display device can becompleted, and display can be performed by application of an electricfield to the microcapsules. For example, the active matrix substrateformed using the thin film transistor in Embodiment 1 can be used.

Note that the first particles and the second particles in themicrocapsules may be formed using one of a conductive material, aninsulating material, a semiconductor material, a magnetic material, aliquid crystal material, a ferroelectric material, an electroluminescentmaterial, an electrochromic material, and a magnetophoretic material ora composite material of any of these materials.

FIG. 13 illustrates active matrix electronic paper as an example of asemiconductor device. A thin film transistor 581 used for thesemiconductor device can be formed in a manner similar to the thin filmtransistor described in Embodiment 1 and is a highly reliable thin filmtransistor including an oxide semiconductor layer.

The electronic paper in FIG. 13 is an example of a display device usinga twisting ball display system. The twisting ball display system refersto a method in which spherical particles each colored in black and whiteare arranged between a first electrode layer and a second electrodelayer which are electrode layers used for a display element, and anpotential difference is generated between the first electrode layer andthe second electrode layer to control the orientation of the sphericalparticles, whereby display is performed.

The thin film transistor 581 formed over a substrate 580 is abottom-gate thin film transistor and covered with an insulating layer583 that is in contact with a semiconductor layer. A source electrodelayer or a drain electrode layer of the thin film transistor 581 is incontact with a first electrode layer 587 through an opening formed inthe insulating layer 583 and an insulating layer 585, whereby the thinfilm transistor 581 is electrically connected to the first electrodelayer 587. Spherical particles 589 are provided between the firstelectrode layer 587 and a second electrode layer 588 formed on asubstrate 596. Each of the spherical particles 589 includes a blackregion 590 a, a white region 590 b, and a cavity 594 filled with liquidaround the black region 590 a and the white region 590 b. A space aroundthe spherical particles 589 is filled with a filler 595 such as a resin(see FIG. 13). The first electrode layer 587 corresponds to a pixelelectrode, and the second electrode layer 588 corresponds to a commonelectrode. The second electrode layer 588 is electrically connected to acommon potential line provided over the substrate where the thin filmtransistor 581 is formed. With the use of a common connection portion,the second electrode layer 588 and the common potential line can beelectrically connected to each other through conductive particlesprovided between the pair of substrates.

Alternatively, it is possible to use an electrophoretic element insteadof the twisting ball. A microcapsule having a diameter of approximately10 μm to 200 μm, in which transparent liquid, positively charged whitemicroparticles, and negatively charged black microparticles areencapsulated, is used. In the microcapsule provided between the firstelectrode layer and the second electrode layer, when an electric fieldis applied by the first electrode layer and the second electrode layer,the white microparticles and the black microparticles move in oppositedirections, whereby white or black can be displayed. A display elementusing this principle is an electrophoretic display element. Theelectrophoretic display element has higher reflectance than a liquidcrystal display element; thus, an auxiliary light is not needed, powerconsumption is low, and a display portion can be recognized in a dimplace. In addition, even when power is not supplied to the displayportion, an image which has been displayed can be maintained. Thus, adisplayed image can be stored.

Through the above-described steps, a highly reliable electronic papercan be manufactured as a semiconductor device.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

Embodiment 5

An example of a light-emitting display device will be described as asemiconductor device. As a display element included in a display device,a light-emitting element utilizing electroluminescence is describedhere. Light-emitting elements utilizing electroluminescence areclassified according to whether a light-emitting material is an organiccompound or an inorganic compound. In general, the former is referred toas an organic EL element and the latter is referred to as an inorganicEL element.

In an organic EL element, by application of voltage to thelight-emitting element, electrons and holes are separately injected froma pair of electrodes into a layer containing a light-emitting organiccompound, and thus current flows. The carriers (electrons and holes) arerecombined, and thus the light-emitting organic compound is excited.When the light-emitting organic compound returns to a ground state fromthe excited state, light is emitted. Owing to such a mechanism, thislight-emitting element is referred to as a current-excitationlight-emitting element.

Inorganic EL elements are classified according to their elementstructures into a dispersion-type inorganic EL element and a thin-filminorganic EL element. A dispersion-type inorganic EL element includes alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission that utilizes a donorlevel and an acceptor level. A thin-film inorganic EL element has astructure where a light-emitting layer is sandwiched between dielectriclayers, which are further sandwiched between electrodes, and its lightemission mechanism is localized type light emission that utilizesinner-shell electron transition of metal ions. Note that here, anorganic EL element is described as a light-emitting element.

FIG. 14 illustrates an example of a pixel structure to which digitaltime grayscale driving can be applied, as an example of a semiconductordevice.

A structure and operation of a pixel to which the digital time grayscaledriving can be applied are described. Here, one pixel includes twon-channel transistors in each of which an oxide semiconductor layer isused for a channel formation region.

A pixel 6400 includes a switching transistor 6401, a transistor 6402, alight-emitting element 6404, and a capacitor 6403. A gate of theswitching transistor 6401 is connected to a scan line 6406. A firstelectrode (one of a source electrode and a drain electrode) of theswitching transistor 6401 is connected to a signal line 6405. A secondelectrode (the other of the source electrode and the drain electrode) ofthe switching transistor 6401 is connected to a gate of the drivingtransistor 6402. The gate of the driving transistor 6402 is connected toa power supply line 6407 through the capacitor 6403. A first electrodeof the driving transistor 6402 is connected to the power supply line6407. A second electrode of the driving transistor 6402 is connected toa first electrode (a pixel electrode) of the light-emitting element6404. A second electrode of the light-emitting element 6404 correspondsto a common electrode 6408. The common electrode 6408 is electricallyconnected to a common potential line provided over the same substrate.

The second electrode (the common electrode 6408) of the light-emittingelement 6404 is set to a low power supply potential. Note that the lowpower supply potential is lower than a high power supply potential thatis set to the power supply line 6407. For example, GND or 0 V may be setas the low power supply potential. A potential difference between thehigh power supply potential and the low power supply potential isapplied to the light-emitting element 6404 so that current flows throughthe light-emitting element 6404, whereby the light-emitting element 6404emits light. In order to make the light-emitting element 6404 emitlight, each potential is set so that the potential difference betweenthe high power supply potential and the low power supply potential ishigher than or equal to the forward threshold voltage of thelight-emitting element 6404.

Note that gate capacitance of the driving transistor 6402 may be used asa substitute for the capacitor 6403, in which case the capacitor 6403can be omitted. The gate capacitance of the driving transistor 6402 maybe formed between the channel region and the gate electrode.

In the case of employing a voltage-input voltage-driving method, a videosignal is input to the gate of the driving transistor 6402 so that thedriving transistor 6402 is in either of two states of being sufficientlyturned on or turned off. In other words, the driving transistor 6402operates in a linear region. Since the driving transistor 6402 operatesin the linear region, a voltage higher than the voltage of the powersupply line 6407 is applied to the gate of the driving transistor 6402.Note that a voltage higher than or equal to the sum of the power supplyline voltage and V_(th) of the driving transistor 6402 is applied to thesignal line 6405.

In the case of employing an analog grayscale method instead of thedigital time grayscale method, the same pixel structure as in FIG. 14can be employed by changing signal input.

In the case of performing analog grayscale driving, a voltage higherthan or equal to the sum of the forward voltage of the light-emittingelement 6404 and V_(th) of the driving transistor 6402 is applied to thegate of the driving transistor 6402. The forward voltage of thelight-emitting element 6404 refers to a voltage at which a desiredluminance is obtained, and refers to at least a forward thresholdvoltage. The video signal by which the driving transistor 6402 operatesin a saturation region is input, so that current can be supplied to thelight-emitting element 6404. In order to operate the driving transistor6402 in the saturation region, the potential of the power supply line6407 is set higher than the gate potential of the driving transistor6402. When an analog video signal is used, a current corresponding tothe video signal is supplied to the light-emitting element 6404, wherebythe analog grayscale driving can be performed.

Note that the pixel structure is not limited to that illustrated in FIG.14. For example, a switch, a resistor, a capacitor, a transistor, alogic circuit, or the like may be added to the pixel illustrated in FIG.14.

Next, a structure of a light-emitting element will be described withreference to FIGS. 15A to 15C. Here, a cross-sectional structure of apixel will be described using an n-channel driving TFT as an example.TFTs 7001, 7011, and 7021 which are driving TFTs used in semiconductordevices illustrated in FIGS. 15A, 15B, and 15C can be formed in a mannersimilar to that of the thin film transistor described in Embodiment 1and are highly reliable thin film transistors each including an oxidesemiconductor layer.

In order to extract light emitted from the light-emitting element, atleast one of an anode and a cathode needs to be transparent. A thin filmtransistor and a light-emitting element are formed over a substrate. Thelight-emitting element can have a top emission structure in which lightis extracted through the surface opposite to the substrate; a bottomemission structure in which light is extracted through the surface onthe substrate side; or a dual emission structure in which light isextracted through the surface opposite to the substrate and the surfaceon the substrate side. The pixel structure can be applied to alight-emitting element having any of these emission structures.

A light-emitting element having the bottom emission structure will bedescribed with reference to FIG. 15A.

FIG. 15A is a cross-sectional view of a pixel in the case where thedriving TFT 7011 is an n-channel TFT and light is emitted from alight-emitting element 7012 to the cathode 7013 side. In FIG. 15A, thecathode 7013 of the light-emitting element 7012 is formed over alight-transmitting conductive layer 7017 that is electrically connectedto the driving TFT 7011, and an EL layer 7014 and an anode 7015 arestacked in this order over the cathode 7013. Note that thelight-transmitting conductive layer 7017 is electrically connected to adrain electrode layer 7030 of the TFT 7011 through a contact hole formedin an oxide insulating layer 7031.

As the light-transmitting conductive layer 7017, a light-transmittingconductive layer formed using indium oxide containing tungsten oxide,indium zinc oxide containing tungsten oxide, indium oxide containingtitanium oxide, indium tin oxide containing titanium oxide, indium tinoxide (hereinafter referred to as ITO), indium zinc oxide, indium tinoxide to which silicon oxide is added, or the like can be used.

Any of a variety of materials can be used for the cathode 7013.Specifically, the cathode 7013 is preferably formed using a materialhaving a low work function such as an alkali metal such as Li or Cs; analkaline earth metal such as Mg, Ca, or Sr; an alloy containing any ofthese metals (e.g., Mg:Ag or Al:Li); or a rare earth metal such as Yb orEr. In FIG. 15A, the thickness of the cathode 7013 is a thickness thatallows light transmission (the thickness is preferably about 5 nm to 30nm). For example, an aluminum layer with a thickness of 20 nm is usedfor the cathode 7013.

Note that a light-transmitting conductive layer and an aluminum layermay be stacked and then selectively etched so that thelight-transmitting conductive layer 7017 and the cathode 7013 areformed. In that case, the etching can be performed using one mask, whichis preferable.

The peripheral portion of the cathode 7013 is covered with a partition7019. The partition 7019 is formed using an organic resin layer ofpolyimide, an acrylic resin, polyamide, an epoxy resin, or the like; aninorganic insulating layer; or organic polysiloxane. It is particularlypreferable that the partition 7019 be formed using a photosensitiveresin material to have an opening over the cathode 7013 so that asidewall of the opening is formed as an inclined surface with continuouscurvature. In the case where a photosensitive resin material is used forthe partition 7019, a step of forming a resist mask can be omitted.

The EL layer 7014 which is formed over the cathode 7013 and thepartition 7019 may be formed using a single layer or a plurality oflayers stacked. When the EL layer 7014 is formed using a plurality oflayers, the EL layer 7014 is formed by stacking an electron-injectionlayer, an electron-transport layer, a light-emitting layer, ahole-transport layer, and a hole-injection layer in that order over thecathode 7013. Note that not all of these layers need to be provided.

The stacking order is not limited to the above order. A hole-injectionlayer, a hole-transport layer, a light-emitting layer, anelectron-transport layer, and an electron-injection layer may be stackedin that order over the cathode 7013. However, when power consumption iscompared, an electron-injection layer, an electron-transport layer, alight-emitting layer, a hole-transport layer, and a hole-injection layerare preferably stacked in that order over the cathode 7013 because oflower power consumption.

A variety of materials can be used for the anode 7015 which is formedover the EL layer 7014. Specifically, a material having a high workfunction, such as titanium nitride, ZrN, Ti, W, Ni, Pt, or Cr; or atransparent conductive material such as ITO, IZO (indium zinc oxide), orZnO, is preferable. In addition, a light-blocking film 7016, forexample, a metal which blocks light, a metal which reflects light, orthe like is provided over the anode 7015. In this embodiment, an ITOfilm is used for the anode 7015, and a Ti layer is used for thelight-blocking film 7016.

The light-emitting element 7012 corresponds to a region where the ELlayer 7014 is sandwiched between the cathode 7013 and the anode 7015. Inthe case of the element structure illustrated in FIG. 15A, light isemitted from the light-emitting element 7012 to the cathode 7013 side asindicated by an arrow.

In FIG. 15A, an example in which a light-transmitting conductive layeris used as a gate electrode layer is illustrated. Light emitted from thelight-emitting element 7012 passes through a color filter layer 7033 andthen passes through the gate electrode layer, a source electrode layer,a drain electrode layer, and the like of the TFT 7011 so as to beemitted to the outside. A light-transmitting conductive layer is used asthe gate electrode layer, the source electrode layer, the drainelectrode layer, and the like of the TFT 7011, whereby aperture ratiocan be improved.

The color filter layer 7033 is formed by a droplet discharge method suchas an inkjet method, a printing method, an etching method with the useof a photolithography technique, or the like.

The color filter layer 7033 is covered with an overcoat layer 7034, andalso covered with a protective insulating layer 7035. Note that,although the overcoat layer 7034 with a thin thickness is illustrated inFIG. 15A, the overcoat layer 7034 has a function of planarizing asurface with unevenness caused by the color filter layer 7033.

Further, a contact hole which is formed in the protective insulatinglayer 7035, the overcoat layer 7034, the color filter layer 7033, andthe oxide insulating layer 7031 and reaches the drain electrode layer7030 is positioned in a region overlapping with the partition 7019. Thecontact hole which reaches the drain electrode layer 7030 and thepartition 7019 overlap with each other, whereby aperture ratio can beimproved.

Next, a light-emitting element having a dual emission structure will bedescribed with reference to FIG. 15B.

In FIG. 15B, a cathode 7023 of a light-emitting element 7022 is formedover a light-transmitting conductive layer 7027 which is electricallyconnected to a TFT 7021, and an EL layer 7024 and an anode 7025 arestacked in that order over the cathode 7023. Note that thelight-transmitting conductive layer 7027 is electrically connected to adrain electrode layer 7040 of the TFT 7021 through a contact hole formedin an oxide insulating layer 7041.

As the light-transmitting conductive layer 7027, a light-transmittingconductive layer formed of indium oxide containing tungsten oxide,indium zinc oxide containing tungsten oxide, indium oxide containingtitanium oxide, indium tin oxide containing titanium oxide, indium tinoxide (hereinafter referred to as ITO), indium zinc oxide, indium tinoxide to which silicon oxide is added, or the like can be used.

Any of a variety of materials can be used for the cathode 7023.Specifically, the cathode 7023 is preferably formed using a materialhaving a low work function such as an alkali metal such as Li or Cs; analkaline earth metal such as Mg, Ca, or Sr; an alloy containing any ofthese metals (e.g., Mg:Ag or Al:Li); or a rare earth metal such as Yb orEr. In this embodiment, the thickness of the cathode 7023 is a thicknessthat allows light transmission (the thickness is preferably about 5 nmto 30 nm). For example, an aluminum layer with a thickness of 20 nm isused for the cathode 7023.

Note that a light-transmitting conductive layer and an aluminum layermay be stacked and then selectively etched so that thelight-transmitting conductive layer 7027 and the cathode 7023 areformed. In that case, the etching can be performed using one mask, whichis preferable.

The peripheral portion of the cathode 7023 is covered with a partition7029. The partition 7029 is formed using an organic resin layer ofpolyimide, an acrylic resin, polyamide, an epoxy resin, or the like; aninorganic insulating layer; or organic polysiloxane. It is particularlypreferable that the partition 7029 be formed using a photosensitiveresin material to have an opening over the cathode 7023 so that asidewall of the opening is formed as an inclined surface with continuouscurvature. In the case where a photosensitive resin material is used forthe partition 7029, a step of forming a resist mask can be omitted.

The EL layer 7024 which is formed over the cathode 7023 and thepartition 7029 may be formed using a single layer or a plurality oflayers stacked. When the EL layer 7024 is formed using a plurality oflayers, the EL layer 7024 is formed by stacking an electron-injectionlayer, an electron-transport layer, a light-emitting layer, ahole-transport layer, and a hole-injection layer in that order over thecathode 7023. Note that not all of these layers need to be provided.

The stacking order is not limited to the above; that is, ahole-injection layer, a hole-transport layer, a light-emitting layer, anelectron-transport layer, and an electron-injection layer may be stackedin that order over the cathode 7023. However, when power consumption iscompared, an electron-injection layer, an electron-transport layer, alight-emitting layer, a hole-transport layer, and a hole-injection layerare preferably stacked in that order over the cathode 7023 because oflower power consumption.

A variety of materials can be used for the anode 7025 which is formedover the EL layer 7024. Specifically, a material having a high workfunction, such as a transparent conductive material such as ITO, IZO, orZnO, is preferable. In this embodiment, an ITO film containing siliconoxide is used for an anode 7026.

The light-emitting element 7022 corresponds to a region where the ELlayer 7024 is sandwiched between the cathode 7023 and the anode 7025. Inthe case of the element structure illustrated in FIG. 15B, light isemitted from the light-emitting element 7022 to both the anode 7025 sideand the cathode 7023 side as indicated by arrows. In FIG. 15B, anexample in which a light-transmitting conductive layer is used as a gateelectrode layer is illustrated. Light emitted from the light-emittingelement 7022 to the cathode 7023 side passes through a color filterlayer 7043 and then passes through the gate electrode layer, a sourceelectrode layer, a drain electrode layer, and the like of the TFT 7011so as to be emitted to the outside. A light-transmitting conductivelayer is used as the gate electrode layer, the source electrode layer,the drain electrode layer, and the like of the TFT 7021, wherebyaperture ratio on the anode 7025 side can be substantially the same asthat on the cathode 7023 side.

The color filter layer 7043 is formed by a droplet discharge method suchas an inkjet method, a printing method, an etching method with the useof a photolithography technique, or the like.

The color filter layer 7043 is covered with an overcoat layer 7044, andalso covered with a protective insulating layer 7045.

Note that when a light-emitting element having a dual emission structureis used and full color display is performed on both display surfaces,light from the anode 7025 side does not pass through the color filterlayer 7043; thus, a sealing substrate provided with another color filterlayer is preferably provided on the anode 7025.

Further, a contact hole which is formed in the protective insulatinglayer 7045, the overcoat layer 7044, the color filter layer 7043, andthe oxide insulating layer 7041 and reaches the drain electrode layer7040 is positioned in a region overlapping with the partition 7029. Thecontact hole which reaches the drain electrode layer 70430 and thepartition 7029 overlap with each other, whereby aperture ratio on theanode 7025 side can be substantially the same as that on the cathode7023 side.

Next, a light-emitting element having a top emission structure will bedescribed with reference to FIG. 15C.

FIG. 15C is a cross-sectional view of a pixel in the case where adriving TFT 7001 is an n-channel thin film transistor and light isemitted from a light-emitting element 7002 to the anode 7005 side. InFIG. 15C, a cathode 7003 of the light-emitting element 7002 which iselectrically connected to the TFT 7001 is formed, and an EL layer 7004and the anode 7005 are stacked in that order over the cathode 7003.

A variety of materials can be used for the cathode 7003. Specifically, amaterial having a low work function, such as an alkali metal such as Lior Cs, an alkaline earth metal such as Mg, Ca, or Sr, an alloycontaining any of these metals (e.g., Mg:Ag or Al:Li), and a rare earthmetal such as Yb or Er, is preferable.

The peripheral portion of the cathode 7003 is covered with a partition7009. The partition 7009 is formed using an organic resin layer ofpolyimide, an acrylic resin, polyamide, an epoxy resin, or the like; aninorganic insulating layer; or organic polysiloxane. It is particularlypreferable that the partition 7009 be formed using a photosensitiveresin material to have an opening over the cathode 7003 so that asidewall of the opening is formed as an inclined surface with continuouscurvature. In the case where a photosensitive resin material is used forthe partition 7009, a step of forming a resist mask can be omitted.

The EL layer 7004 which is formed over the cathode 7003 and thepartition 7009 may be formed using a single layer or a plurality oflayers stacked. When the EL layer 7004 is formed using a plurality oflayers, the EL layer 7004 is formed by stacking an electron-injectionlayer, an electron-transport layer, a light-emitting layer, ahole-transport layer, and a hole-injection layer in that order over thecathode 7003.

Note that not all of these layers need to be provided.

The stacking order is not limited to the above; that is, ahole-injection layer, a hole-transport layer, a light-emitting layer, anelectron-transport layer, and an electron-injection layer may be stackedin that order over the cathode 7003. In the case where these layers arestacked in that order, the cathode 7003 functions as an anode.

In FIG. 15C, a hole-injection layer, a hole-transport layer, alight-emitting layer, an electron-transport layer, and an electroninjection layer are stacked in this order over a stacked layer in whicha Ti layer, an aluminum, a Ti layer are stacked in this order, andthereover, a stacked layer of a Mg:Ag alloy thin film and an ITO film isformed.

However, when power consumption is compared, an electron-injectionlayer, an electron-transport layer, a light-emitting layer, ahole-transport layer, and a hole-injection layer are preferably stackedin that order over the cathode 7003 because of lower power consumption.

The anode 7005 is formed using a light-transmitting conductive material.The anode 7005 may be formed using a light-transmitting conductive layerformed using, for example, indium oxide containing tungsten oxide,indium zinc oxide containing tungsten oxide, indium oxide containingtitanium oxide, indium tin oxide containing titanium oxide, indium tinoxide, indium zinc oxide, or indium tin oxide to which silicon oxide isadded.

The light-emitting element 7002 corresponds to a region where the ELlayer 7004 is sandwiched between the cathode 7003 and the anode 7005. Inthe case of the element structure illustrated in FIG. 15C, light isemitted from the light-emitting element 7002 to the anode 7005 side asindicated by an arrow.

In FIG. 15C, the example in which the thin film transistor 150 is usedas the TFT 7001 is illustrated; however, there is no particularlimitation thereto, and the thin film transistor 160, the thin filmtransistor 170, or the thin film transistor 180 can be used.

In FIG. 15C, the cathode 7003 is electrically connected to a drainelectrode layer 7050 of the TFT 7001 through a contact hole formed in anoxide insulating layer 7051, a planarization insulating layer 7053, andan insulating layer 7055. The planarization insulating layer 7053 can beformed using a resin material such as polyimide, an acrylic resin, abenzocyclobutene-based resin, polyamide, or an epoxy resin. Besides theabove-described resin materials, it is also possible to use alow-dielectric constant material (low-k material), a siloxane-basedresin, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), orthe like. Note that the planarization insulating layer 7053 may beformed by stacking a plurality of insulating layers formed of thesematerials. There is no particular limitation on the method for formingthe planarization insulating layer 7053. The planarization insulatinglayer 7053 can be formed, depending on the material, by a method such asa sputtering method, an SOG method, spin coating, dip coating, spraycoating, or a droplet discharge method (such as an inkjet method, screenprinting, offset printing, or the like), or a tool such as a doctorknife, a roll coater, a curtain coater, or a knife coater.

The partition 7009 is provided so as to insulate the cathode 7003 from acathode 7008 of an adjacent pixel. The partition 7009 is formed using anorganic resin layer formed of polyimide, an acrylic resin, polyamide, anepoxy resin, or the like; an inorganic insulating layer; or organicpolysiloxane. It is particularly preferable that the partition 7009 beformed using a photosensitive resin material to have an opening over thecathode 7003 so that a sidewall of the opening is formed as an inclinedsurface with continuous curvature. In the case where a photosensitiveresin material is used for the partition 7009, a step of forming aresist mask can be omitted.

In the structure of FIG. 15C, when full color display is performed, forexample, the light-emitting element 7002 is used as a greenlight-emitting element, one of adjacent light-emitting elements is usedas a red light-emitting element, and the other is used as a bluelight-emitting element. Alternatively, a light-emitting display devicecapable of full color display may be manufactured using four kinds oflight-emitting elements which include a white light-emitting element inaddition to three kinds of light-emitting elements.

In the structure of FIG. 15C, a light-emitting display device capable offull color display may be manufactured in such a manner that all of aplurality of light-emitting elements which is arranged is whitelight-emitting elements and a sealing substrate having a color filter orthe like is arranged on the light-emitting element 7002. A materialwhich exhibits a single color such as white is formed and combined witha color filter or a color conversion layer, whereby full color displaycan be performed.

Needless to say, display of monochromatic light can also be performed.For example, a lighting system may be formed with the use of white lightemission, or an area-color light-emitting device may be formed with theuse of a single color light emission.

If necessary, an optical film such as a polarizing film including acircularly polarizing plate may be provided.

Note that although the organic EL elements are described here as thelight-emitting elements, an inorganic EL element can be provided as thelight-emitting element.

Note that the example is described in which the thin film transistor(the driving TFT) which controls the driving of a light-emitting elementis electrically connected to the light-emitting element; alternatively,a structure may be employed in which a TFT for current control isconnected between the driving TFT and the light-emitting element.

Next, the appearance and a cross section of a light-emitting displaypanel (also referred to as a light-emitting panel) which is oneembodiment of a semiconductor device will be described with reference toFIGS. 16A and 16B. FIG. 16A is a plan view of a panel in which a thinfilm transistor and a light-emitting element that are formed over afirst substrate are sealed between the first substrate and a secondsubstrate with a sealant. FIG. 16B is a cross-sectional view taken alongline H-I in FIG. 16A.

A sealant 4505 is provided so as to surround a pixel portion 4502,signal line driver circuits 4503 a and 4503 b, and scan line drivercircuits 4504 a and 4504 b which are provided over a first substrate4501. Moreover, a second substrate 4506 is provided over the pixelportion 4502, the signal line driver circuits 4503 a and 4503 b, and thescan line driver circuits 4504 a and 4504 b. Consequently, the pixelportion 4502, the signal line driver circuits 4503 a and 4503 b, and thescan line driver circuits 4504 a and 4504 b are sealed together with afiller 4507 by the first substrate 4501, the sealant 4505, and thesecond substrate 4506. In this manner, a panel is preferably packaged(sealed) with a protection film (such as a laminate film or anultraviolet curable resin film) or a cover material with highair-tightness and little degasification so that the panel is not exposedto the outside air.

The pixel portion 4502, the signal line driver circuits 4503 a and 4503b, and the scan line driver circuits 4504 a and 4504 b, which are formedover the first substrate 4501, each include a plurality of thin filmtransistors. A thin film transistor 4510 included in the pixel portion4502 and a thin film transistor 4509 included in the signal line drivercircuit 4503 a are illustrated as an example in FIG. 16B.

The highly reliable thin film transistor including the oxidesemiconductor layer described in Embodiment 1 can be used as the thinfilm transistors 4509 and 4510. In this embodiment, the thin filmtransistors 4509 and 4510 are n-channel thin film transistors.

A conductive layer 4540 is provided over part of an insulating layer4544, which overlaps with a channel formation region of an oxidesemiconductor layer in the thin film transistor 4509 for the drivercircuit. The conductive layer 4540 is provided at the positionoverlapping with the channel formation region of the oxide semiconductorlayer, whereby the amount of change in the threshold voltage of the thinfilm transistor 4509 before and after the BT test can be reduced. Thepotential of the conductive layer 4540 may be the same or different fromthat of a gate electrode layer in the thin film transistor 4509. Theconductive layer 4540 can also function as a second gate electrodelayer. Alternatively, the potential of the conductive layer 4540 may beGND or 0 V, or the conductive layer 4540 may be in a floating state.

In the thin film transistor 4509, as a protective insulating layer, aninsulating layer 4541 is formed in contact with a semiconductor layerincluding a channel formation region. The insulating layer 4541 may beformed using a material and a method which are similar to those of theinsulating layer 107 described in Embodiment 1. Moreover, the insulatinglayer 4544 functioning as a planarization insulating layer covers thethin film transistors in order to reduce surface unevenness of the thinfilm transistors. Here, as the insulating layer 4541, a silicon oxidelayer is formed by a sputtering method in a manner similar to that ofthe insulating layer 107 described in Embodiment 1.

Further, a protective insulating layer 4543 is formed over theinsulating layer 4541. The protective insulating layer 4543 may beformed using a material and a method similar to those of the protectiveinsulating layer 407 described in Embodiment 1. Here, as the protectiveinsulating layer 4543, a silicon nitride film is formed by a PCVDmethod.

Further, the insulating layer 4544 is formed as the planarizationinsulating layer. The insulating layer 4544 may be formed using amaterial and a method which are similar to those of the insulating layer4021 described in Embodiment 3. Here, the insulating layer 4544 isformed using an acrylic resin.

Reference numeral 4511 denotes a light-emitting element. A firstelectrode layer 4517 which is a pixel electrode included in thelight-emitting element 4511 is electrically connected to a sourceelectrode layer or a drain electrode layer of the thin film transistor4510. Note that the structure of the light-emitting element 4511 is notlimited to a layered structure of the first electrode layer 4517, anelectroluminescent layer 4512, and a second electrode layer 4513. Thestructure of the light-emitting element 4511 can be changed asappropriate depending on the direction in which light is extracted fromthe light-emitting element 4511, or the like.

A partition 4520 is formed using an organic resin layer, an inorganicinsulating layer, or organic polysiloxane. It is particularly preferablethat the partition 4520 be formed using a photosensitive material and anopening be formed over the first electrode layer 4517 so that a sidewallof the opening is formed as an inclined surface with continuouscurvature.

The electroluminescent layer 4512 may be formed as a single layer or aplurality of layers stacked.

A protection layer may be formed over the second electrode layer 4513and the partition 4520 in order to prevent entry of oxygen, hydrogen,moisture, carbon dioxide, or the like into the light-emitting element4511. As the protection layer, a silicon nitride layer, a siliconnitride oxide layer, a DLC layer, or the like can be formed.

In addition, a variety of signals and potentials are supplied to thesignal line driver circuits 4503 a and 4503 b, the scan line drivercircuits 4504 a and 4504 b, or the pixel portion 4502 from FPCs 4518 aand 4518 b.

A connection terminal electrode 4515 is formed using the same conductivelayer as the first electrode layer 4517 included in the light-emittingelement 4511, and a terminal electrode 4516 is formed using the sameconductive layer as the source and drain electrode layers included inthe thin film transistors 4509 and 4510.

The connection terminal electrode 4515 is electrically connected to aterminal included in the FPC 4518 a through an anisotropic conductivelayer 4519.

The substrate positioned in the direction in which light is extractedfrom the light-emitting element 4511 needs to have light-transmittingproperties. In that case, a light-transmitting material such as a glassplate, a plastic plate, a polyester film, or an acrylic resin film isused for the second substrate.

As the filler 4507, an ultraviolet curable resin or a thermosettingresin can be used in addition to an inert gas such as nitrogen or argon.For example, polyvinyl chloride (PVC), an acrylic resin, polyimide, anepoxy resin, a silicone resin, polyvinyl butyral (PVB), or ethylenevinyl acetate (EVA) can be used. For example, nitrogen is used for thefiller

If needed, an optical film such as a polarizing plate, a circularlypolarizing plate (including an elliptically polarizing plate), aretardation plate (a quarter-wave plate or a half-wave plate), or acolor filter may be provided as appropriate on a light-emitting surfaceof the light-emitting element. Further, a polarizing plate or acircularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment can be performed by whichreflected light can be diffused by projections and depressions on thesurface so as to reduce glare.

Driver circuits formed using a single crystal semiconductor or apolycrystalline semiconductor over a substrate separately prepared maybe mounted as the signal line driver circuits 4503 a and 4503 b and thescan line driver circuits 4504 a and 4504 b. Alternatively, only thesignal line driver circuits or part thereof, or only the scan linedriver circuits or part thereof may be separately formed and mounted.This embodiment is not limited to the structure illustrated in FIGS. 16Aand 16B.

Through the above-described steps, a highly reliable light-emittingdisplay device (display panel) can be manufactured as a semiconductordevice.

The thin film transistors in the pixel portion of the light-emittingdisplay device are manufactured by the manufacturing method of the thinfilm transistor which is described in Embodiment 1, whereby displayunevenness caused by variation in threshold voltage of the thin filmtransistors in pixels can be suppressed.

In addition, the thin film transistors in the driver circuits of thelight-emitting display device are manufactured by the manufacturingmethod of the thin film transistor which is described in Embodiment 1,whereby high-speed operation of the thin film transistors in the drivercircuit portion can be realized and power saving can be achieved.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

Embodiment 6

In this embodiment, as one embodiment of a semiconductor device, anexample of a liquid crystal display device including a liquid crystalelement and the thin film transistor described in Embodiment 1 will bedescribed with reference to FIGS. 17, 18, 19, and 20. The thin filmtransistor described in Embodiment 1 can be used as TFTs 628 and 629used in a liquid crystal display device illustrated in FIGS. 17, 18, 19,and 20. The TFTs 628 and 629 are transistors with excellent electriccharacteristics and high reliability which can be manufactured through aprocess similar to that described in Embodiment 1. The TFTs 628 and 629each include a channel formation region formed in an oxide semiconductorlayer. With reference to FIGS. 17, 18, 19, and 20, a case where the thinfilm transistor illustrated in FIG. 3C is used as an example of a thinfilm transistor will be described; however, this embodiment is notlimited thereto.

A vertical alignment (VA) liquid crystal display device will bedescribed below. VA liquid crystal display device has a kind of form inwhich alignment of liquid crystal molecules of a liquid crystal displaypanel is controlled. In the VA liquid crystal display device, liquidcrystal molecules are aligned in a vertical direction with respect to apanel surface when no voltage is applied. In this embodiment, inparticular, a pixel is divided into some regions (subpixels), andmolecules are aligned in different directions in their respectiveregions. This is referred to as multi-domain or multi-domain design.Liquid crystal display devices of the multi-domain design will bedescribed below.

FIG. 18 and FIG. 19 illustrate a pixel electrode and a counterelectrode, respectively. FIG. 18 is a plan view showing the substrateside where the pixel electrode is formed. FIG. 17 illustrates across-sectional structure taken along section line E-F in FIG. 18. FIG.19 is a plan view showing the substrate side where the counter electrodeis formed. Description below is made with reference to those drawings.

In FIG. 17, a substrate 600 over which a TFT 628, a pixel electrode 624connected to the TFT 628, and a storage capacitor portion 630 are formedand a counter substrate 601 provided with a counter electrode 640 andthe like overlap with each other, and liquid crystals are injectedbetween the substrate 600 and the counter substrate 601.

Although not illustrated, a columnar spacer which is higher than aprojection 644 is formed between the substrate 600 and the countersubstrate 601 to make a distance (a cell gap) between the pixelelectrode 624 and the counter electrode 640 constant. An alignment film648 is formed over the pixel electrode 624. In a similar manner, thecounter electrode 640 is provided with an alignment film 646. A liquidcrystal layer 650 is formed between the alignment films 646 and 648.

Although a columnar spacer is used for the spacer here, bead spacers maybe dispersed instead. Further, the spacer may also be formed over thepixel electrode 624 provided over the substrate 600.

The TFT 628, the pixel electrode 624 connected to the TFT 628, and thestorage capacitor portion 630 are formed over the substrate 600. Thepixel electrode 624 is connected to a wiring 618 through a contact hole623 which penetrates an insulating layer 620 for covering the TFT 628, awiring 616, and the storage capacitor portion 630 and also penetrates aninsulating layer 622 for covering the insulating layer 620. The thinfilm transistor described in Embodiment 1 can be used as the TFT 628 asappropriate. Further, the storage capacitor portion 630 includes a firstcapacitor wiring 604 which is formed at the same time as a gate wiring602 of the TFT 628; a gate insulating layer 606; and a second capacitorwiring 617 which is formed at the same time as the wirings 616 and 618.

The pixel electrode 624, the liquid crystal layer 650, and the counterelectrode 640 overlap with each other, whereby a liquid crystal elementis formed.

FIG. 18 illustrates a planar structure on the substrate 600. The pixelelectrode 624 can be formed using a light-transmitting conductivematerial such as indium oxide containing tungsten oxide, indium zincoxide containing tungsten oxide, indium oxide containing titanium oxide,indium tin oxide containing titanium oxide, indium tin oxide(hereinafter referred to as ITO), indium zinc oxide, or indium tin oxideto which silicon oxide is added.

The pixel electrode 624 can be formed using a conductive compositionincluding a conductive macromolecule (also referred to as a conductivepolymer). The pixel electrode formed using the conductive compositionpreferably has a sheet resistance of less than or equal to 10000 ohmsper square and a light transmittance of greater than or equal to 70% ata wavelength of 550 nm. Further, the resistivity of the conductive highmolecule included in the conductive composition is preferably less thanor equal to 0.1 Ω·cm.

As the conductive high molecule, a so-called π-electron conjugatedconductive polymer can be used. As examples, polyaniline and aderivative thereof, polypyrrole and a derivative thereof, polythiopheneand a derivative thereof, and a copolymer of two or more kinds of thesematerials are given.

The pixel electrode 624 is provided with slits 625. The slits 625 areprovided for controlling the alignment of the liquid crystals.

A TFT 629, a pixel electrode 626 connected to the TFT 629, and a storagecapacitor portion 631 which are illustrated in FIG. 18 can be formed ina similar manner to the TFT 628, the pixel electrode 624, and thestorage capacitor portion 630, respectively. Both the TFTs 628 and 629are connected to the wiring 616. One pixel of this liquid crystaldisplay panel includes the pixel electrodes 624 and 626. The pixelelectrodes 624 and 626 constitute subpixels.

FIG. 19 illustrates a planar structure of the counter substrate side.The counter electrode 640 is preferably formed using a material similarto that of the pixel electrode 624. The protrusions 644 which controlthe alignment of the liquid crystals are formed on the counter electrode640.

FIG. 20 illustrates an equivalent circuit of this pixel structure. Boththe TFTs 628 and 629 are connected to the gate wiring 602 and the wiring616. In that case, when potentials of the capacitor wiring 604 and acapacitor wiring 605 are different from each other, operations of liquidcrystal elements 651 and 652 can vary. In other words, the alignment ofthe liquid crystals is precisely controlled and a viewing angle isincreased by separate control of potentials of the capacitor wirings 604and 605.

When a voltage is applied to the pixel electrode 624 provided with theslits 625, a distorted electric field (an oblique electric field) isgenerated in the vicinity of the slits 625. The protrusions 644 on thecounter substrate 601 side and the slits 625 are alternately arranged sothat the oblique electric field is effectively generated to control thealignment of the liquid crystals, whereby the direction of the alignmentof the liquid crystals varies depending on the location. In other words,a viewing angle of the liquid crystal display panel is increased bymulti-domain.

Next, a VA liquid crystal display device, which is different from theabove-described device, will be described with reference to FIGS. 21 to24.

FIG. 21 and FIG. 22 illustrate a pixel structure of a VA liquid crystaldisplay panel. FIG. 22 is a plan view of the substrate 600. FIG. 21illustrates a cross-sectional structure taken along section line Y-Z inFIG. 22. Description below will be given with reference to both thedrawings.

In this pixel structure, a plurality of pixel electrodes are provided inone pixel, and a TFT is connected to each of the pixel electrodes. Theplurality of TFTs are driven by different gate signals. In other words,signals applied to individual pixel electrodes in a multi-domain pixelare controlled independently of each other.

The pixel electrode 624 is connected to the TFT 628 through a wiring 618in the contact hole 623. The pixel electrode 626 is connected to the TFT629 through a wiring 619 in a contact hole 627. A gate wiring 602 of theTFT 628 is separated from a gate wiring 603 of the TFT 629 so thatdifferent gate signals can be supplied. On the other hand, a wiring 616serving as a data line is shared by the TFTs 628 and 629. The thin filmtransistor described in Embodiment 1 can be used as appropriate as eachof the TFTs 628 and 629. In addition, a capacitor wiring 690 isprovided.

The shape of the pixel electrode 624 is different from that of the pixelelectrode 626, and the pixel electrode 626 is formed so as to surroundthe external side of the pixel electrode 624 which spreads into a Vshape. A voltage applied to the pixel electrode 624 by a TFT 628 is madeto be different from a voltage applied to the pixel electrode 626 by aTFT 629, whereby alignment of liquid crystals is controlled. FIG. 24illustrates an equivalent circuit of this pixel structure. The TFT 628is connected to the gate wiring 602, and the TFT 629 is connected to thegate wiring 603. Both the TFTs 628 and 629 are connected to the wiring616. A signal supplied to the gate wiring 602 and a signal supplied tothe gate wiring 603 are separately controlled, whereby voltage appliedto the liquid crystal element 651 and voltage applied to the liquidcrystal element 652 can be different from each other. In other words,the operations of the TFTs 628 and 629 are controlled separately toprecisely control the alignment of the liquid crystals in the liquidcrystal elements 651 and 652, which leads to a wider viewing angle. Inother words, the operations of the TFTs 628 and 629 are separatelycontrolled, whereby the alignment of liquid crystals vary in the liquidcrystal element 651 and the liquid crystal element 652 is realized,which leads to a wider viewing angle.

A counter substrate 601 is provided with a coloring layer 636 and acounter electrode 640. A planarization layer 637 is formed between thecoloring layer 636 and the counter electrode 640 to prevent alignmentdisorder of the liquid crystals. FIG. 23 illustrates a structure of thecounter substrate side. The counter electrode 640 is an electrode sharedby different pixels and slits 641 are formed. The slits 641 and theslits 625 on the pixel electrode 624 side and the pixel electrode 626side are alternately arranged so that an oblique electric field iseffectively generated, whereby the alignment of the liquid crystals canbe controlled. Accordingly, the alignment of the liquid crystals canvary in different locations, which leads to a wider viewing angle. Notethat in FIG. 23, the pixel electrodes 624 and 626 formed over thesubstrate 600 are represented by dashed lines and the counter electrode640 and the pixel electrodes 624 and 626 overlap with each other.

An alignment film 648 is formed over the pixel electrode 624 and thepixel electrode 626, and the counter electrode 640 is similarly providedwith an alignment film 646. A liquid crystal layer 650 is formed betweenthe substrate 600 and a counter substrate 601.

The pixel electrode 624, the liquid crystal layer 650, and the counterelectrode 640 overlap with each other, whereby a first liquid crystalelement is formed. Further, the pixel electrode 626, the liquid crystallayer 650, and the counter electrode 640 overlap with each other,whereby a second liquid crystal element is formed. The pixel structureof the display panel illustrated in FIGS. 21, 22, 23, and 24 is amulti-domain structure in which the first liquid crystal element and thesecond liquid crystal element are provided in one pixel.

Although the VA liquid crystal display device is described as a liquidcrystal display device including the thin film transistor described inEmbodiment 1, an IPS liquid crystal display device, a TN liquid crystaldisplay device, or the like can be used.

Thin film transistors in a pixel portion of the liquid crystal displaydevice are manufactured by the manufacturing method of the thin filmtransistor which is described in Embodiment 1, whereby displayunevenness caused by variation in threshold voltage of the thin filmtransistors in pixels can be suppressed.

Embodiment 7

A semiconductor device disclosed in this specification can be applied toelectronic paper. Electronic paper can be used for electronic devices inall fields as long as they display data. For example, electronic papercan be applied to an e-book reader (an electronic book), a poster, anadvertisement in a vehicle such as a train, or displays of a variety ofcards such as a credit card. FIG. 25 illustrates an example of theelectronic devices.

FIG. 25 illustrates an e-book reader 2700. For example, the e-bookreader 2700 includes two housings of a housing 2701 and a housing 2703.The housing 2701 and the housing 2703 are combined with a hinge 2711 sothat the e-book reader 2700 can be opened and closed with the hinge 2711as an axis. Such a structure enables the e-book reader 2700 to operatelike a paper book.

A display portion 2705 and a display portion 2707 are incorporated inthe housing 2701 and the housing 2703, respectively. The display portion2705 and the display portion 2707 may display one image or differentimages. In the case where the display portion 2705 and the displayportion 2707 display different images, for example, a display portion onthe right side (the display portion 2705 in FIG. 20) can display textand a display portion on the left side (the display portion 2707 in FIG.20) can display graphics.

FIG. 25 illustrates an example in which the housing 2701 is providedwith an operation portion and the like. For example, the housing 2701 isprovided with a power switch 2721, operation keys 2723, a speaker 2725,and the like. Pages can be turned with the operation keys 2723. Notethat a keyboard, a pointing device, and the like may be provided on thesame surface as the display portion of the housing. Moreover, anexternal connection terminal (an earphone terminal, a USB terminal, aterminal connectable to a variety of cables such as an AC adapter and aUSB cable, or the like), a recording medium insertion portion, and thelike may be provided on the back surface or the side surface of thehousing. Moreover, the e-book reader 2700 may have a function of anelectronic dictionary.

The e-book reader 2700 may be configured to wirelessly transmit andreceive data. Through wireless communication, desired book data or thelike can be purchased and downloaded from an electronic book server.

Embodiment 8

A semiconductor device disclosed in this specification can be applied toa variety of electronic devices (including game machines). Examples ofsuch electronic devices are a television set (also referred to as atelevision or a television receiver), a monitor of a computer or thelike, a camera such as a digital camera or a digital video camera, adigital photo frame, a mobile phone handset (also referred to as amobile phone or a mobile phone device), a portable game machine, aportable information terminal, an audio reproducing device, alarge-sized game machine such as a pinball machine, and the like.

FIG. 26A illustrates an example of television set. In a television set9600, a display portion 9603 is incorporated in a housing 9601. Thedisplay portion 9603 can display images. Here, the housing 9601 issupported by a stand 9605.

The television set 9600 can be operated with an operation switch of thehousing 9601 or a separate remote controller 9610. Channels can beswitched and volume can be controlled with operation keys 9609 of theremote controller 9610, whereby an image displayed on the displayportion 9603 can be controlled. Moreover, the remote controller 9610 maybe provided with a display portion 9607 for displaying data outputtedfrom the remote controller 9610.

Note that the television set 9600 is provided with a receiver, a modem,and the like. With the use of the receiver, general TV broadcasts can bereceived. Moreover, when the display device is connected to acommunication network with or without wires via the modem, one-way (froma sender to a receiver) or two-way (between a sender and a receiver orbetween receivers) information communication can be performed.

FIG. 26B illustrates an example of digital photo frame. For example, ina digital photo frame 9700, a display portion 9703 is incorporated in ahousing 9701. The display portion 9703 can display a variety of images.For example, the display portion 9703 can display data of an image takenwith a digital camera or the like and function as a normal photo frame.

Note that the digital photo frame 9700 is provided with an operationportion, an external connection terminal (a USB terminal, a terminalconnectable to a variety of cables such as a USB cable, or the like), arecording medium insertion portion, and the like. Although thesecomponents may be provided on the same surface as the display portion,it is preferable to provide them on the side surface or the back surfacefor design aesthetics. For example, a memory storing data of an imagetaken with a digital camera is inserted in the recording mediuminsertion portion of the digital photo frame 9700 and the data isloaded, whereby the image can be displayed on the display portion 9703.

The digital photo frame 9700 may be configured to transmit and receivedata wirelessly. Through wireless communication, desired image data canbe loaded to be displayed.

FIG. 27A is a portable game machine and is constituted by two housingsof a housing 9881 and a housing 9891 which are connected with a jointportion 9893 so that the portable game machine can be opened or folded.A display portion 9882 and a display portion 9883 are incorporated inthe housing 9881 and the housing 9891, respectively. In addition, theportable game machine illustrated in FIG. 27A is provided with a speakerportion 9884, a recording medium insertion portion 9886, an LED lamp9890, input means (operation keys 9885, a connection terminal 9887, asensor 9888 (having a function of measuring force, displacement,position, speed, acceleration, angular velocity, rotation number,distance, light, liquid, magnetism, temperature, chemical substance,sound, time, hardness, electric field, current, voltage, electric power,radial ray, flow rate, humidity, gradient, vibration, smell, or infraredray), and a microphone 9889), and the like. Needless to say, thestructure of the portable game machine is not limited to the above andother structures provided with at least a semiconductor device disclosedin this specification can be employed. The portable game machine mayinclude an additional accessory as appropriate. The portable gamemachine illustrated in FIG. 27A has a function of reading a program ordata stored in the recording medium to display it on the displayportion, and a function of sharing data with another portable gamemachine by wireless communication. Note that a function of the portablegame machine illustrated in FIG. 27A is not limited to those describedabove, and the portable game machine can have a variety of functions.

FIG. 27B illustrates a slot machine 9900 which is a large-sized gamemachine. In the slot machine 9900, a display portion 9903 isincorporated in a housing 9901. In addition, the slot machine 9900includes an operation means such as a start lever or a stop switch, acoin slot, a speaker, and the like. Needless to say, the structure ofthe slot machine 9900 is not limited to the above and other structuresprovided with at least a semiconductor device disclosed in thisspecification may be employed. The slot machine 9900 may include anadditional accessory as appropriate.

FIG. 28A is a perspective view illustrating an example of portablecomputer.

In the portable computer illustrated in FIG. 28A, a top housing 9301having a display portion 9303 and a bottom housing 9302 having akeyboard 9304 can overlap with each other by closing a hinge unit whichconnects the top housing 9301 and the bottom housing 9302. Thus, theportable computer is conveniently carried.

Moreover, in the case of using the keyboard for input of data, the hingeunit is opened so that a user can input data looking at the displayportion 9303.

The bottom housing 9302 includes a pointing device 9306 with which inputcan be performed, in addition to the keyboard 9304. When the displayportion 9303 is a touch panel, a user can input data by touching part ofthe display portion. The bottom housing 9302 includes an arithmeticfunction portion such as a CPU or hard disk. In addition, the bottomhousing 9302 includes an external connection port 9305 into whichanother device, for example, a communication cable conformable tocommunication standards of a USB is inserted.

The top housing 9301 further includes a display portion 9307 which canbe stowed in the top housing 9301 by being slid therein. With thedisplay portion 9307, a large display screen can be realized. Inaddition, the user can adjust the angle of a screen of the stowabledisplay portion 9307. If the stowable display portion 9307 is a touchpanel, the user can input data by touching part of the display portion9307.

The display portion 9303 or the stowable display portion 9307 is formedusing an image display device such as a liquid crystal display panel ora light-emitting display panel using an organic light-emitting element,an inorganic light-emitting element, or the like.

In addition, the portable computer illustrated in FIG. 28A can beprovided with a receiver and the like and can receive a TV broadcast todisplay an image on the display portion 9303 or the display portion9307. The user can watch a TV broadcast with the whole screen of thedisplay portion 9307 by sliding and exposing the display portion 9307and adjusting the angle thereof, with the hinge unit which connects thetop housing 9301 and the bottom housing 9302 closed. In this case, thehinge unit is not opened and display is not performed on the displayportion 9303. In addition, start up of only a circuit for displaying aTV broadcast is performed. Thus, power consumption can be minimized,which is useful for the portable computer whose battery capacity islimited.

FIG. 28B is a perspective view of an example of a portable phone thatthe user can wear on the wrist like a wristwatch.

This mobile phone is formed with a main body which includes acommunication device including at least a telephone function, and abattery; a band portion 9204 which enables the main body to be worn onthe wrist; an adjusting portion 9205 for adjusting the band portion 9204to fit the wrist; a display portion 9201; a speaker 9207; and amicrophone 9208.

In addition, the main body includes operation switches 9203. Theoperation switches 9203 can serve, for example, as a switch for startinga program for the Internet when pushed, in addition to serving as apower switch, a switch for switching displays, a switch for instructionto start taking images, or the like, and can be configured to haverespective functions.

A user can input data into this mobile phone by touching the displayportion 9201 with a finger or an input pen, operating the operationswitches 9203, or inputting voice into the microphone 9208. In FIG. 28B,display buttons 9202 are displayed on the display portion 9201. A usercan input data by touching the display buttons 9202 with a finger or thelike.

Further, the main body includes a camera portion 9206 including an imagepick-up means having a function of converting an image of an object,which is formed through a camera lens, to an electronic image signal.Note that the camera portion is not necessarily provided.

The portable phone illustrated in FIG. 28B is provided with a receiverof a TV broadcast and the like, and can display an image on the displayportion 9201 by receiving the TV broadcast. In addition, the mobilephone is provided with a memory device such as a memory, and the like,and can record the TV broadcast in the memory. The portable phoneillustrated in FIG. 28B may have a function of collecting locationinformation such as GPS.

An image display device such as a liquid crystal display panel or alight-emitting display panel using an organic light-emitting element, aninorganic light-emitting element, or the like is used as the displayportion 9201. The portable phone illustrated in FIG. 28B is compact andlightweight and the battery capacity is limited. For the above reason, apanel which can be driven with low power consumption is preferably usedas a display device for the display portion 9201.

Note that, although FIG. 28B illustrates the electronic device which isworn on the wrist, this embodiment is not limited thereto as long as anelectronic is portable.

Example 1

In this example, a thin film transistor was manufactured by themanufacturing method which is described in Embodiment 1. Evaluationresults of the amount of change in V_(th) before and after a BT test areshown.

In this example, thin film transistors each of which had a channellength L of 3 μm and a channel width W of 20 μm were manufactured overone substrate. First, a method for manufacturing the thin filmtransistor is described.

First, as a base layer, by a CVD method, a silicon nitride layer wasformed over a glass substrate and a silicon oxynitride layer was formedover the silicon nitride layer. Over the silicon oxynitride layer, atungsten layer was formed as a gate electrode layer by a sputteringmethod. Here, the tungsten layer was selectively etched so that the gateelectrode layer was formed.

Next, over the gate electrode layer, a silicon oxynitride layer with athickness of 100 nm was formed as a gate insulating layer by a CVDmethod.

Next, over the gate insulating layer, an oxide semiconductor layer witha thickness of 30 nm was formed using an In—Ga—Zn—O-based oxidesemiconductor target (In₂O₃:Ga₂O₃:ZnO=1:1:1 in a molar ratio) having arelative density of an oxide semiconductor of 85% by a sputteringmethod. Here, the oxide semiconductor layer was selectively etched sothat an island-shaped oxide semiconductor layer was formed.

Next, first heat treatment was performed on the oxide semiconductorlayer at 650° C. for 6 minutes in a nitrogen atmosphere by a GRTAmethod.

Next, as a source and drain electrode layers, a 100-nm-thick titaniumlayer, a 200-nm-thick aluminum layer, and a 100-nm-thick titanium layerwere stacked over the oxide semiconductor layer by a sputtering method.Here, the source and drain electrode layers were selectively etched sothat the channel length L and the channel width W of the thin filmtransistor were 3 μm and 20 μm, respectively.

Next, as a protective insulating layer, a silicon oxide layer was formedso as to be in contact with the oxide semiconductor layer by a reactivesputtering method. Here, the silicon oxide layer that was the protectivelayer was selectively etched so that an opening was formed over the gateelectrode layer and the source and drain electrode layers. After that,second heat treatment was performed at 250° C. for one hour in anitrogen atmosphere.

Through the above-described steps, the thin film transistor wasmanufactured.

Subsequently, a BT test was performed on the thin film transistormanufactured in this example. The BT test is a kind of accelerationtest, by which a change in characteristics of the thin film transistorthat is caused by long-term use can be evaluated in a short time. Inparticular, the amount of change in threshold voltage of the thin filmtransistor before and after the BT test is an important indicator forexamining reliability. A smaller amount of change in the thresholdvoltage of the thin film transistor before and after the BT test meanshigh reliability of the thin film transistor.

Specifically, while the temperature of a substrate (substratetemperature) over which a thin film transistor is formed is maintainedat a constant temperature and potentials of a source and a drain of thethin film transistor are set to the same potential, a potential which isdifferent from that of the source and drain is applied to a gate of thethin film transistor for a certain period of time. The substratetemperature may be set as appropriate in accordance with the testpurpose. A BT test in which a potential applied to a gate is higher thanpotentials of a source and a drain is referred to as a +BT test and a BTtest in which a potential applied to a gate is lower than potentials ofa source and a drain is referred to as a −BT test.

The stress conditions for the BT test can be determined in accordancewith a substrate temperature, intensity of electric field applied to agate insulating layer, and a time period of application of electricfield. The intensity of the electric field applied to the gateinsulating layer is determined in accordance with a value obtained bydividing a potential difference between the gate and the source anddrain by the thickness of the gate insulating layer. For example, in thecase where the intensity of the electric field applied to the gateinsulating layer with a thickness of 100 nm is to be 2 MV/cm, thepotential difference may be set to 20 V.

Note that voltage refers to a difference in potential between twopoints, and potential refers to electrostatic energy (potential energy)of a unit charge at a given point in an electrostatic field. Note thatin general, a difference between potential of one point and referencepotential is merely called potential or voltage, and potential andvoltage are used as synonymous words in many cases. Therefore, in thisspecification, potential may be rephrased as voltage and voltage may berephrased as potential unless otherwise specified.

Both the +BT test and the −BT test were performed under the followingconditions: the substrate temperature was 150° C.; the intensity of anelectric field applied to the gate insulating layer was 2 MV/cm; and thetime for application was one hour.

First, the +BT test is described. In order to measure initialcharacteristics of thin film a transistor subjected to the BT test, achange in characteristics of source-drain current (hereinafter referredto as drain current or I_(d)) was measured under conditions that thesubstrate temperature was set to 40° C., the voltage between a sourceand a drain (hereinafter, referred to as drain voltage or V_(d)) was setto 10 V, and the voltage between the source and a gate (hereinafterreferred to as gate voltage or V_(g)) was changed from −20 V to +20 V.In other words, V_(g)-I_(d) characteristics were measured. Here, as acountermeasure against moisture absorption onto surfaces of a sample,the substrate temperature was set to 40° C. However, the measurement maybe performed at room temperature (25° C.) if there is no particularproblem.

Next, after the substrate temperature was increased to 150° C., thepotential of the source and drain of the thin film transistor was set to0 V. Then, voltage was applied to the gate so that the intensity of anelectric field applied to the gate insulating layer was 2 MV/cm. Sincethe thickness of the gate insulating layer in the thin film transistorwas 100 nm here, a voltage of +20 V was kept being applied to the gatefor one hour. Although the time for voltage application was one hourhere, the time may be determined as appropriate in accordance with thepurpose.

Next, the substrate temperature was decreased to 40° C. while voltagewas continuously applied to the gate and the source and drain. In thatcase, if the application of voltage is stopped before the substratetemperature is completely decreased, damage which is given to the thinfilm transistor in the BT test is repaired due to remaining heat; thus,the substrate temperature needs to be decreased while voltage isapplied. After the substrate temperature was decreased to 40° C., theapplication of the voltage was stopped. Strictly, the time taken fortemperature drop needs to be added to the time for voltage application;however, the temperature was actually able to be decreased to 40° C. inseveral minutes, and therefore this is taken as an error range and thetime taken for temperature drop is not added to the time for voltageapplication.

Then, V_(g)-I_(d) characteristics were measured under the sameconditions as those for the measurement of the initial characteristics,so that the V_(g)-I_(d) characteristics after the +BT test wereobtained.

Next, the −BT test is described. The −BT test was performed with theprocedure similar to the +BT test, but has a different point from the+BT test, in that the voltage applied to the gate after the substratetemperature is increased to 150° C. is set to −20 V.

Note that it is important that a BT test be performed on a thin filmtransistor which has never been subjected to a BT test. For example,when a −BT test is performed on a thin film transistor which has alreadybeen subjected to a +BT test, results of the −BT test cannot beevaluated correctly due to the influence of the +BT test which has beenperformed earlier. Further, the same can be said for the case where a+BT test is performed again on a thin film transistor which has alreadybeen subjected to a +BT test. Note that the same cannot be said for thecase where a BT test is intentionally repeated in consideration of suchan influence.

Here, the definition of V_(th) in this specification is described. InFIG. 29, the horizontal axis represents gate voltage on a linear scaleand the vertical axis represents square root of drain current(hereinafter also referred to as √I_(d)) on a linear scale. A curve 501represents square root of drain current with respect to a change in thegate voltage and is a curve (hereinafter also referred to as a √I_(d)curve) representing square root of I_(d) of a V_(g)-I_(d) curve measuredunder such a condition that V_(d) was 10 V.

First, the √I_(d) curve (the curve 501) is obtained from the V_(g)-I_(d)curve measured under such a condition that V_(d) was 10 V. Then, atangent 504 of a point on the √I_(d) curve at which a differential valueof the √I_(d) curve becomes maximum is obtained. Next, V_(g) at the timewhen the tangent 504 is extended and I_(d) becomes 0 A on the tangent504, that is, the value of gate voltage axis intercept 505 is defined asV_(th).

FIGS. 30A and 30B each show V_(g)-I_(d) characteristics of the thin filmtransistor before and after the BT test. In both FIGS. 30A and 30B, thehorizontal axis represents gate voltage (V_(g)) and the vertical axisrepresents drain current (I_(d)) with respect to the gate voltage on alogarithmic scale.

FIG. 30A shows the V_(g)-I_(d) characteristics of the thin filmtransistor before and after the +BT test. Initial characteristics 331represents the V_(g)-I_(d) characteristics of the thin film transistorbefore the +BT test and +BT 332 represents the V_(g)-I_(d)characteristics of the thin film transistor after the +BT test.

FIG. 30B shows the V_(g)-I_(d) characteristics of the thin filmtransistor before and after the −BT test. Initial characteristics 341represent the V_(g)-I_(d) characteristics of the thin film transistorbefore the −BT test, and −BT 342 represents the V_(g)-I_(d)characteristics of the thin film transistor after the −BT test.

In FIG. 30A, the threshold voltage of the +BT 332 is shifted from thatof the initial characteristics 331 in a positive direction by 0.72 V. InFIG. 30B, the threshold voltage of the −BT 342 is shifted from that ofthe initial characteristics 341 in a positive direction by 0.04 V. Inboth the BT tests, the amount of change in the threshold voltage is lessthan or equal to 1 V, which confirms that the thin film transistormanufactured in accordance with Embodiment 1 has high reliability.

Here, results of a BT test performed on a conventional thin filmtransistor are described. The conventional thin film transistor is achannel-etched thin film transistor like the thin film transistor inEmbodiment 1. Although not particularly illustrated, in the conventionalthin film transistor, a base insulating layer is formed over a glasssubstrate; a gate electrode layer is formed over the base insulatinglayer; a silicon oxynitride layer with a thickness of 100 nm is formedas a gate insulating layer over the gate electrode layer by a CVDmethod; an oxide semiconductor layer with a thickness of 20 nm is formedover the gate insulating layer with the use of an In—Ga—Zn—O-based oxidesemiconductor target (In₂O₃:Ga₂O₃:ZnO=1:1:1 in a molar ratio) having arelative density of an oxide semiconductor of 85% by a sputteringmethod; a source and drain electrodes are formed over the oxidesemiconductor layer; and as a protective insulating layer, a siliconoxynitride layer is formed over the source and drain electrodes so as tobe in contact with part of the oxide semiconductor layer by a CVDmethod. Note that first heat treatment is not performed on theconventional thin film transistor.

FIGS. 31A and 31B each show V_(g)-I_(d) characteristics of theconventional thin film transistor before and after the BT test. In themeasurement of the V_(g)-I_(d) characteristics of the conventional thinfilm transistor used in this example, I_(d) was less than or equal to alower detection limit of a measurement device in an off region (ingeneral, a region where V_(g) ranges from about 0 V to negative voltagein the case of an n-channel transistor). Therefore, FIGS. 31A and 31B donot show a part in which I_(d) is less than or equal to the lowerdetection limit of the measurement device.

FIG. 31A shows the V_(g)-I_(d) characteristics of the conventional thinfilm transistor before and after the +BT test. Initial characteristics311 represents the V_(g)-I_(d) characteristics of the conventional thinfilm transistor before the +BT test and +BT 312 represents theV_(g)-I_(d) characteristics after the +BT test.

FIG. 31B shows the V_(g)-I_(d) characteristics of the conventional thinfilm transistor before and after the −BT test. Initial characteristics321 represents the V_(g)-I_(d) characteristics of the conventional thinfilm transistor before the −BT test and −BT 322 represents theV_(g)-I_(d) characteristics after the −BT test.

In FIG. 31A, the threshold voltage of the +BT 312 is shifted from thatof the initial characteristics 311 in a positive direction by 5.7 V. InFIG. 31B, the threshold voltage of the −BT 322 is shifted from that ofthe initial characteristics 321 in a negative direction by 3.4 V.

This application is based on Japanese Patent Application serial no.2009-218904 filed with Japan Patent Office on Sep. 24, 2009, the entirecontents of which are hereby incorporated by reference.

1. (canceled)
 2. A semiconductor device comprising: a pixel over asubstrate; and a driver circuit over the substrate, the driver circuitelectrically connected to the pixel through a scan line, wherein thepixel comprises a first transistor and a light-emitting elementelectrically connected to the first transistor, wherein the drivercircuit comprises a second transistor, wherein each of the firsttransistor and the second transistor comprises: a source electrode and adrain electrode over the substrate; an oxide semiconductor layer overthe source electrode and the drain electrode; a first gate electrodeover the oxide semiconductor layer; and a gate insulating film betweenthe oxide semiconductor layer and the first gate electrode, wherein thefirst transistor has a single-gate structure, wherein the secondtransistor further comprises a second gate electrode overlapping theoxide semiconductor layer of the second transistor, and wherein thefirst gate electrode of the second transistor and the second gateelectrode of the second transistor are at the same potential.
 3. Thesemiconductor device according to claim 2, wherein the oxidesemiconductor layer comprises indium, gallium, and zinc.
 4. Thesemiconductor device according to claim 2, wherein the first gateelectrode comprises a first titanium layer, an aluminum layer over thefirst titanium layer, and a second titanium layer over the aluminumlayer.
 5. A semiconductor device comprising: a pixel over a substrate;and a driver circuit over the substrate, the driver circuit electricallyconnected to the pixel through a scan line, wherein the pixel comprisesa first transistor and a light-emitting element electrically connectedto the first transistor, wherein the driver circuit comprises a secondtransistor, wherein each of the first transistor and the secondtransistor comprises: a source electrode and a drain electrode over thesubstrate; an oxide semiconductor layer over the source electrode andthe drain electrode; a first gate electrode over the oxide semiconductorlayer; and a gate insulating film between the oxide semiconductor layerand the first gate electrode, wherein the first transistor has asingle-gate structure, and wherein the second transistor furthercomprises a second gate electrode overlapping the oxide semiconductorlayer of the second transistor.
 6. The semiconductor device according toclaim 5, wherein the oxide semiconductor layer comprises indium,gallium, and zinc.
 7. The semiconductor device according to claim 5,wherein the first gate electrode comprises a first titanium layer, analuminum layer over the first titanium layer, and a second titaniumlayer over the aluminum layer.